tree dca5a8340ef4b0f2cbfeb33dc984d5e41d67d7b3
parent 56ef7bdc6a7d7d2f354ee5baaa2fae7a374a0958
author Alexandru Elisei <alexandru.elisei@arm.com> 1589470698 +0100
committer Will Deacon <will@kernel.org> 1589906281 +0100

ioport: mmio: Use a mutex and reference counting for locking

kvmtool uses brlock for protecting accesses to the ioport and mmio
red-black trees. brlock allows concurrent reads, but only one writer, which
is assumed not to be a VCPU thread (for more information see commit
0b907ed2eaec ("kvm tools: Add a brlock)). This is done by issuing a
compiler barrier on read and pausing the entire virtual machine on writes.
When KVM_BRLOCK_DEBUG is defined, brlock uses instead a pthread read/write
lock.

When we will implement reassignable BARs, the mmio or ioport mapping will
be done as a result of a VCPU mmio access. When brlock is a pthread
read/write lock, it means that we will try to acquire a write lock with the
read lock already held by the same VCPU and we will deadlock. When it's
not, a VCPU will have to call kvm__pause, which means the virtual machine
will stay paused forever.

Let's avoid all this by using a mutex and reference counting the red-black
tree entries. This way we can guarantee that we won't unregister a node
that another thread is currently using for emulation.

Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Link: https://lore.kernel.org/r/1589470709-4104-2-git-send-email-alexandru.elisei@arm.com
Signed-off-by: Will Deacon <will@kernel.org>