Documentation/virt/kvm/locking.rst - linux - Git at Google

 .. SPDX-License-Identifier: GPL-2.0

 =================
 KVM Lock Overview
 =================

 1. Acquisition Orders
 ---------------------

 The acquisition orders for mutexes are as follows:

 - cpus_read_lock() is taken outside kvm_lock

 - kvm->lock is taken outside vcpu->mutex

 - kvm->lock is taken outside kvm->slots_lock and kvm->irq_lock

 - kvm->slots_lock is taken outside kvm->irq_lock, though acquiring
   them together is quite rare.

 - kvm->mn_active_invalidate_count ensures that pairs of
   invalidate_range_start() and invalidate_range_end() callbacks
   use the same memslots array.  kvm->slots_lock and kvm->slots_arch_lock
   are taken on the waiting side when modifying memslots, so MMU notifiers
   must not take either kvm->slots_lock or kvm->slots_arch_lock.

 For SRCU:

 - ``synchronize_srcu(&kvm->srcu)`` is called inside critical sections
   for kvm->lock, vcpu->mutex and kvm->slots_lock.  These locks _cannot_
   be taken inside a kvm->srcu read-side critical section; that is, the
   following is broken::

       srcu_read_lock(&kvm->srcu);
       mutex_lock(&kvm->slots_lock);

 - kvm->slots_arch_lock instead is released before the call to
   ``synchronize_srcu()``.  It _can_ therefore be taken inside a
   kvm->srcu read-side critical section, for example while processing
   a vmexit.

 On x86:

 - vcpu->mutex is taken outside kvm->arch.hyperv.hv_lock and kvm->arch.xen.xen_lock

 - kvm->arch.mmu_lock is an rwlock; critical sections for
   kvm->arch.tdp_mmu_pages_lock and kvm->arch.mmu_unsync_pages_lock must
   also take kvm->arch.mmu_lock

 Everything else is a leaf: no other lock is taken inside the critical
 sections.

 2. Exception
 ------------

 Fast page fault:

 Fast page fault is the fast path which fixes the guest page fault out of
 the mmu-lock on x86. Currently, the page fault can be fast in one of the
 following two cases:

 1. Access Tracking: The SPTE is not present, but it is marked for access
    tracking. That means we need to restore the saved R/X bits. This is
    described in more detail later below.

 2. Write-Protection: The SPTE is present and the fault is caused by
    write-protect. That means we just need to change the W bit of the spte.

 What we use to avoid all the races is the Host-writable bit and MMU-writable bit
 on the spte:

 - Host-writable means the gfn is writable in the host kernel page tables and in
   its KVM memslot.
 - MMU-writable means the gfn is writable in the guest's mmu and it is not
   write-protected by shadow page write-protection.

 On fast page fault path, we will use cmpxchg to atomically set the spte W
 bit if spte.HOST_WRITEABLE = 1 and spte.WRITE_PROTECT = 1, to restore the saved
 R/X bits if for an access-traced spte, or both. This is safe because whenever
 changing these bits can be detected by cmpxchg.

 But we need carefully check these cases:

 1) The mapping from gfn to pfn

 The mapping from gfn to pfn may be changed since we can only ensure the pfn
 is not changed during cmpxchg. This is a ABA problem, for example, below case
 will happen:

 +------------------------------------------------------------------------+
 | At the beginning::                                                     |
 |                                                                        |
 |	gpte = gfn1                                                      |
 |	gfn1 is mapped to pfn1 on host                                   |
 |	spte is the shadow page table entry corresponding with gpte and  |
 |	spte = pfn1                                                      |
 +------------------------------------------------------------------------+
 | On fast page fault path:                                               |
 +------------------------------------+-----------------------------------+
 | CPU 0:                             | CPU 1:                            |
 +------------------------------------+-----------------------------------+
 | ::                                 |                                   |
 |                                    |                                   |
 |   old_spte = *spte;                |                                   |
 +------------------------------------+-----------------------------------+
 |                                    | pfn1 is swapped out::             |
 |                                    |                                   |
 |                                    |    spte = 0;                      |
 |                                    |                                   |
 |                                    | pfn1 is re-alloced for gfn2.      |
 |                                    |                                   |
 |                                    | gpte is changed to point to       |
 |                                    | gfn2 by the guest::               |
 |                                    |                                   |
 |                                    |    spte = pfn1;                   |
 +------------------------------------+-----------------------------------+
 | ::                                                                     |
 |                                                                        |
 |   if (cmpxchg(spte, old_spte, old_spte+W)                              |
 |	mark_page_dirty(vcpu->kvm, gfn1)                                 |
 |            OOPS!!!                                                     |
 +------------------------------------------------------------------------+

 We dirty-log for gfn1, that means gfn2 is lost in dirty-bitmap.

 For direct sp, we can easily avoid it since the spte of direct sp is fixed
 to gfn.  For indirect sp, we disabled fast page fault for simplicity.

 A solution for indirect sp could be to pin the gfn, for example via
 kvm_vcpu_gfn_to_pfn_atomic, before the cmpxchg.  After the pinning:

 - We have held the refcount of pfn; that means the pfn can not be freed and
   be reused for another gfn.
 - The pfn is writable and therefore it cannot be shared between different gfns
   by KSM.

 Then, we can ensure the dirty bitmaps is correctly set for a gfn.

 2) Dirty bit tracking

 In the origin code, the spte can be fast updated (non-atomically) if the
 spte is read-only and the Accessed bit has already been set since the
 Accessed bit and Dirty bit can not be lost.

 But it is not true after fast page fault since the spte can be marked
 writable between reading spte and updating spte. Like below case:

 +------------------------------------------------------------------------+
 | At the beginning::                                                     |
 |                                                                        |
 |	spte.W = 0                                                       |
 |	spte.Accessed = 1                                                |
 +------------------------------------+-----------------------------------+
 | CPU 0:                             | CPU 1:                            |
 +------------------------------------+-----------------------------------+
 | In mmu_spte_clear_track_bits()::   |                                   |
 |                                    |                                   |
 |  old_spte = *spte;                 |                                   |
 |                                    |                                   |
 |                                    |                                   |
 |  /* 'if' condition is satisfied. */|                                   |
 |  if (old_spte.Accessed == 1 &&     |                                   |
 |       old_spte.W == 0)             |                                   |
 |     spte = 0ull;                   |                                   |
 +------------------------------------+-----------------------------------+
 |                                    | on fast page fault path::         |
 |                                    |                                   |
 |                                    |    spte.W = 1                     |
 |                                    |                                   |
 |                                    | memory write on the spte::        |
 |                                    |                                   |
 |                                    |    spte.Dirty = 1                 |
 +------------------------------------+-----------------------------------+
 |  ::                                |                                   |
 |                                    |                                   |
 |   else                             |                                   |
 |     old_spte = xchg(spte, 0ull)    |                                   |
 |   if (old_spte.Accessed == 1)      |                                   |
 |     kvm_set_pfn_accessed(spte.pfn);|                                   |
 |   if (old_spte.Dirty == 1)         |                                   |
 |     kvm_set_pfn_dirty(spte.pfn);   |                                   |
 |     OOPS!!!                        |                                   |
 +------------------------------------+-----------------------------------+

 The Dirty bit is lost in this case.

 In order to avoid this kind of issue, we always treat the spte as "volatile"
 if it can be updated out of mmu-lock [see spte_has_volatile_bits()]; it means
 the spte is always atomically updated in this case.

 3) flush tlbs due to spte updated

 If the spte is updated from writable to read-only, we should flush all TLBs,
 otherwise rmap_write_protect will find a read-only spte, even though the
 writable spte might be cached on a CPU's TLB.

 As mentioned before, the spte can be updated to writable out of mmu-lock on
 fast page fault path. In order to easily audit the path, we see if TLBs needing
 to be flushed caused this reason in mmu_spte_update() since this is a common
 function to update spte (present -> present).

 Since the spte is "volatile" if it can be updated out of mmu-lock, we always
 atomically update the spte and the race caused by fast page fault can be avoided.
 See the comments in spte_has_volatile_bits() and mmu_spte_update().

 Lockless Access Tracking:

 This is used for Intel CPUs that are using EPT but do not support the EPT A/D
 bits. In this case, PTEs are tagged as A/D disabled (using ignored bits), and
 when the KVM MMU notifier is called to track accesses to a page (via
 kvm_mmu_notifier_clear_flush_young), it marks the PTE not-present in hardware
 by clearing the RWX bits in the PTE and storing the original R & X bits in more
 unused/ignored bits. When the VM tries to access the page later on, a fault is
 generated and the fast page fault mechanism described above is used to
 atomically restore the PTE to a Present state. The W bit is not saved when the
 PTE is marked for access tracking and during restoration to the Present state,
 the W bit is set depending on whether or not it was a write access. If it
 wasn't, then the W bit will remain clear until a write access happens, at which
 time it will be set using the Dirty tracking mechanism described above.

 3. Reference
 ------------

 ``kvm_lock``
 ^^^^^^^^^^^^

 :Type:		mutex
 :Arch:		any
 :Protects:	- vm_list
 		- kvm_usage_count
 		- hardware virtualization enable/disable
 :Comment:	KVM also disables CPU hotplug via cpus_read_lock() during
 		enable/disable.

 ``kvm->mn_invalidate_lock``
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^

 :Type:          spinlock_t
 :Arch:          any
 :Protects:      mn_active_invalidate_count, mn_memslots_update_rcuwait

 ``kvm_arch::tsc_write_lock``
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^

 :Type:		raw_spinlock_t
 :Arch:		x86
 :Protects:	- kvm_arch::{last_tsc_write,last_tsc_nsec,last_tsc_offset}
 		- tsc offset in vmcb
 :Comment:	'raw' because updating the tsc offsets must not be preempted.

 ``kvm->mmu_lock``
 ^^^^^^^^^^^^^^^^^
 :Type:		spinlock_t or rwlock_t
 :Arch:		any
 :Protects:	-shadow page/shadow tlb entry
 :Comment:	it is a spinlock since it is used in mmu notifier.

 ``kvm->srcu``
 ^^^^^^^^^^^^^
 :Type:		srcu lock
 :Arch:		any
 :Protects:	- kvm->memslots
 		- kvm->buses
 :Comment:	The srcu read lock must be held while accessing memslots (e.g.
 		when using gfn_to_* functions) and while accessing in-kernel
 		MMIO/PIO address->device structure mapping (kvm->buses).
 		The srcu index can be stored in kvm_vcpu->srcu_idx per vcpu
 		if it is needed by multiple functions.

 ``kvm->slots_arch_lock``
 ^^^^^^^^^^^^^^^^^^^^^^^^
 :Type:          mutex
 :Arch:          any (only needed on x86 though)
 :Protects:      any arch-specific fields of memslots that have to be modified
                 in a ``kvm->srcu`` read-side critical section.
 :Comment:       must be held before reading the pointer to the current memslots,
                 until after all changes to the memslots are complete

 ``wakeup_vcpus_on_cpu_lock``
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 :Type:		spinlock_t
 :Arch:		x86
 :Protects:	wakeup_vcpus_on_cpu
 :Comment:	This is a per-CPU lock and it is used for VT-d posted-interrupts.
 		When VT-d posted-interrupts are supported and the VM has assigned
 		devices, we put the blocked vCPU on the list blocked_vcpu_on_cpu
 		protected by blocked_vcpu_on_cpu_lock. When VT-d hardware issues
 		wakeup notification event since external interrupts from the
 		assigned devices happens, we will find the vCPU on the list to
 		wakeup.

 ``vendor_module_lock``
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 :Type:		mutex
 :Arch:		x86
 :Protects:	loading a vendor module (kvm_amd or kvm_intel)
 :Comment:	Exists because using kvm_lock leads to deadlock.  cpu_hotplug_lock is
     taken outside of kvm_lock, e.g. in KVM's CPU online/offline callbacks, and
     many operations need to take cpu_hotplug_lock when loading a vendor module,
     e.g. updating static calls.
	.. SPDX-License-Identifier: GPL-2.0

	=================
	KVM Lock Overview
	=================

	1. Acquisition Orders
	---------------------

	The acquisition orders for mutexes are as follows:

	- cpus_read_lock() is taken outside kvm_lock

	- kvm->lock is taken outside vcpu->mutex

	- kvm->lock is taken outside kvm->slots_lock and kvm->irq_lock

	- kvm->slots_lock is taken outside kvm->irq_lock, though acquiring
	them together is quite rare.

	- kvm->mn_active_invalidate_count ensures that pairs of
	invalidate_range_start() and invalidate_range_end() callbacks
	use the same memslots array. kvm->slots_lock and kvm->slots_arch_lock
	are taken on the waiting side when modifying memslots, so MMU notifiers
	must not take either kvm->slots_lock or kvm->slots_arch_lock.

	For SRCU:

	- ``synchronize_srcu(&kvm->srcu)`` is called inside critical sections
	for kvm->lock, vcpu->mutex and kvm->slots_lock. These locks _cannot_
	be taken inside a kvm->srcu read-side critical section; that is, the
	following is broken::

	srcu_read_lock(&kvm->srcu);
	mutex_lock(&kvm->slots_lock);

	- kvm->slots_arch_lock instead is released before the call to
	``synchronize_srcu()``. It _can_ therefore be taken inside a
	kvm->srcu read-side critical section, for example while processing
	a vmexit.

	On x86:

	- vcpu->mutex is taken outside kvm->arch.hyperv.hv_lock and kvm->arch.xen.xen_lock

	- kvm->arch.mmu_lock is an rwlock; critical sections for
	kvm->arch.tdp_mmu_pages_lock and kvm->arch.mmu_unsync_pages_lock must
	also take kvm->arch.mmu_lock

	Everything else is a leaf: no other lock is taken inside the critical
	sections.

	2. Exception
	------------

	Fast page fault:

	Fast page fault is the fast path which fixes the guest page fault out of
	the mmu-lock on x86. Currently, the page fault can be fast in one of the
	following two cases:

	1. Access Tracking: The SPTE is not present, but it is marked for access
	tracking. That means we need to restore the saved R/X bits. This is
	described in more detail later below.

	2. Write-Protection: The SPTE is present and the fault is caused by
	write-protect. That means we just need to change the W bit of the spte.

	What we use to avoid all the races is the Host-writable bit and MMU-writable bit
	on the spte:

	- Host-writable means the gfn is writable in the host kernel page tables and in
	its KVM memslot.
	- MMU-writable means the gfn is writable in the guest's mmu and it is not
	write-protected by shadow page write-protection.

	On fast page fault path, we will use cmpxchg to atomically set the spte W
	bit if spte.HOST_WRITEABLE = 1 and spte.WRITE_PROTECT = 1, to restore the saved
	R/X bits if for an access-traced spte, or both. This is safe because whenever
	changing these bits can be detected by cmpxchg.

	But we need carefully check these cases:

	1) The mapping from gfn to pfn

	The mapping from gfn to pfn may be changed since we can only ensure the pfn
	is not changed during cmpxchg. This is a ABA problem, for example, below case
	will happen:

	+------------------------------------------------------------------------+
	\| At the beginning:: \|
	\| \|
	\| gpte = gfn1 \|
	\| gfn1 is mapped to pfn1 on host \|
	\| spte is the shadow page table entry corresponding with gpte and \|
	\| spte = pfn1 \|
	+------------------------------------------------------------------------+
	\| On fast page fault path: \|
	+------------------------------------+-----------------------------------+
	\| CPU 0: \| CPU 1: \|
	+------------------------------------+-----------------------------------+
	\| :: \| \|
	\| \| \|
	\| old_spte = *spte; \| \|
	+------------------------------------+-----------------------------------+
	\| \| pfn1 is swapped out:: \|
	\| \| \|
	\| \| spte = 0; \|
	\| \| \|
	\| \| pfn1 is re-alloced for gfn2. \|
	\| \| \|
	\| \| gpte is changed to point to \|
	\| \| gfn2 by the guest:: \|
	\| \| \|
	\| \| spte = pfn1; \|
	+------------------------------------+-----------------------------------+
	\| :: \|
	\| \|
	\| if (cmpxchg(spte, old_spte, old_spte+W) \|
	\| mark_page_dirty(vcpu->kvm, gfn1) \|
	\| OOPS!!! \|
	+------------------------------------------------------------------------+

	We dirty-log for gfn1, that means gfn2 is lost in dirty-bitmap.

	For direct sp, we can easily avoid it since the spte of direct sp is fixed
	to gfn. For indirect sp, we disabled fast page fault for simplicity.

	A solution for indirect sp could be to pin the gfn, for example via
	kvm_vcpu_gfn_to_pfn_atomic, before the cmpxchg. After the pinning:

	- We have held the refcount of pfn; that means the pfn can not be freed and
	be reused for another gfn.
	- The pfn is writable and therefore it cannot be shared between different gfns
	by KSM.

	Then, we can ensure the dirty bitmaps is correctly set for a gfn.

	2) Dirty bit tracking

	In the origin code, the spte can be fast updated (non-atomically) if the
	spte is read-only and the Accessed bit has already been set since the
	Accessed bit and Dirty bit can not be lost.

	But it is not true after fast page fault since the spte can be marked
	writable between reading spte and updating spte. Like below case:

	+------------------------------------------------------------------------+
	\| At the beginning:: \|
	\| \|
	\| spte.W = 0 \|
	\| spte.Accessed = 1 \|
	+------------------------------------+-----------------------------------+
	\| CPU 0: \| CPU 1: \|
	+------------------------------------+-----------------------------------+
	\| In mmu_spte_clear_track_bits():: \| \|
	\| \| \|
	\| old_spte = *spte; \| \|
	\| \| \|
	\| \| \|
	\| /* 'if' condition is satisfied. */\| \|
	\| if (old_spte.Accessed == 1 && \| \|
	\| old_spte.W == 0) \| \|
	\| spte = 0ull; \| \|
	+------------------------------------+-----------------------------------+
	\| \| on fast page fault path:: \|
	\| \| \|
	\| \| spte.W = 1 \|
	\| \| \|
	\| \| memory write on the spte:: \|
	\| \| \|
	\| \| spte.Dirty = 1 \|
	+------------------------------------+-----------------------------------+
	\| :: \| \|
	\| \| \|
	\| else \| \|
	\| old_spte = xchg(spte, 0ull) \| \|
	\| if (old_spte.Accessed == 1) \| \|
	\| kvm_set_pfn_accessed(spte.pfn);\| \|
	\| if (old_spte.Dirty == 1) \| \|
	\| kvm_set_pfn_dirty(spte.pfn); \| \|
	\| OOPS!!! \| \|
	+------------------------------------+-----------------------------------+

	The Dirty bit is lost in this case.

	In order to avoid this kind of issue, we always treat the spte as "volatile"
	if it can be updated out of mmu-lock [see spte_has_volatile_bits()]; it means
	the spte is always atomically updated in this case.

	3) flush tlbs due to spte updated

	If the spte is updated from writable to read-only, we should flush all TLBs,
	otherwise rmap_write_protect will find a read-only spte, even though the
	writable spte might be cached on a CPU's TLB.

	As mentioned before, the spte can be updated to writable out of mmu-lock on
	fast page fault path. In order to easily audit the path, we see if TLBs needing
	to be flushed caused this reason in mmu_spte_update() since this is a common
	function to update spte (present -> present).

	Since the spte is "volatile" if it can be updated out of mmu-lock, we always
	atomically update the spte and the race caused by fast page fault can be avoided.
	See the comments in spte_has_volatile_bits() and mmu_spte_update().

	Lockless Access Tracking:

	This is used for Intel CPUs that are using EPT but do not support the EPT A/D
	bits. In this case, PTEs are tagged as A/D disabled (using ignored bits), and
	when the KVM MMU notifier is called to track accesses to a page (via
	kvm_mmu_notifier_clear_flush_young), it marks the PTE not-present in hardware
	by clearing the RWX bits in the PTE and storing the original R & X bits in more
	unused/ignored bits. When the VM tries to access the page later on, a fault is
	generated and the fast page fault mechanism described above is used to
	atomically restore the PTE to a Present state. The W bit is not saved when the
	PTE is marked for access tracking and during restoration to the Present state,
	the W bit is set depending on whether or not it was a write access. If it
	wasn't, then the W bit will remain clear until a write access happens, at which
	time it will be set using the Dirty tracking mechanism described above.

	3. Reference
	------------

	``kvm_lock``
	^^^^^^^^^^^^

	:Type: mutex
	:Arch: any
	:Protects: - vm_list
	- kvm_usage_count
	- hardware virtualization enable/disable
	:Comment: KVM also disables CPU hotplug via cpus_read_lock() during
	enable/disable.

	``kvm->mn_invalidate_lock``
	^^^^^^^^^^^^^^^^^^^^^^^^^^^

	:Type: spinlock_t
	:Arch: any
	:Protects: mn_active_invalidate_count, mn_memslots_update_rcuwait

	``kvm_arch::tsc_write_lock``
	^^^^^^^^^^^^^^^^^^^^^^^^^^^^

	:Type: raw_spinlock_t
	:Arch: x86
	:Protects: - kvm_arch::{last_tsc_write,last_tsc_nsec,last_tsc_offset}
	- tsc offset in vmcb
	:Comment: 'raw' because updating the tsc offsets must not be preempted.

	``kvm->mmu_lock``
	^^^^^^^^^^^^^^^^^
	:Type: spinlock_t or rwlock_t
	:Arch: any
	:Protects: -shadow page/shadow tlb entry
	:Comment: it is a spinlock since it is used in mmu notifier.

	``kvm->srcu``
	^^^^^^^^^^^^^
	:Type: srcu lock
	:Arch: any
	:Protects: - kvm->memslots
	- kvm->buses
	:Comment: The srcu read lock must be held while accessing memslots (e.g.
	when using gfn_to_* functions) and while accessing in-kernel
	MMIO/PIO address->device structure mapping (kvm->buses).
	The srcu index can be stored in kvm_vcpu->srcu_idx per vcpu
	if it is needed by multiple functions.

	``kvm->slots_arch_lock``
	^^^^^^^^^^^^^^^^^^^^^^^^
	:Type: mutex
	:Arch: any (only needed on x86 though)
	:Protects: any arch-specific fields of memslots that have to be modified
	in a ``kvm->srcu`` read-side critical section.
	:Comment: must be held before reading the pointer to the current memslots,
	until after all changes to the memslots are complete

	``wakeup_vcpus_on_cpu_lock``
	^^^^^^^^^^^^^^^^^^^^^^^^^^^^
	:Type: spinlock_t
	:Arch: x86
	:Protects: wakeup_vcpus_on_cpu
	:Comment: This is a per-CPU lock and it is used for VT-d posted-interrupts.
	When VT-d posted-interrupts are supported and the VM has assigned
	devices, we put the blocked vCPU on the list blocked_vcpu_on_cpu
	protected by blocked_vcpu_on_cpu_lock. When VT-d hardware issues
	wakeup notification event since external interrupts from the
	assigned devices happens, we will find the vCPU on the list to
	wakeup.

	``vendor_module_lock``
	^^^^^^^^^^^^^^^^^^^^^^^^^^^^
	:Type: mutex
	:Arch: x86
	:Protects: loading a vendor module (kvm_amd or kvm_intel)
	:Comment: Exists because using kvm_lock leads to deadlock. cpu_hotplug_lock is
	taken outside of kvm_lock, e.g. in KVM's CPU online/offline callbacks, and
	many operations need to take cpu_hotplug_lock when loading a vendor module,
	e.g. updating static calls.