|DAWR issues on POWER9
|On POWER9 the Data Address Watchpoint Register (DAWR) can cause a checkstop
|if it points to cache inhibited (CI) memory. Currently Linux has no way to
|distinguish CI memory when configuring the DAWR, so (for now) the DAWR is
|disabled by this commit::
| commit 9654153158d3e0684a1bdb76dbababdb7111d5a0
| Author: Michael Neuling <firstname.lastname@example.org>
| Date: Tue Mar 27 15:37:24 2018 +1100
| powerpc: Disable DAWR in the base POWER9 CPU features
|DAWR has 6 different ways of being set.
|For ptrace, we now advertise zero breakpoints on POWER9 via the
|PPC_PTRACE_GETHWDBGINFO call. This results in GDB falling back to
|software emulation of the watchpoint (which is slow).
|h_set_mode(DAWR) and h_set_dabr() will now return an error to the
|guest on a POWER9 host. Current Linux guests ignore this error, so
|they will silently not get the DAWR.
|kvmppc_set_one_reg() will store the value in the vcpu but won't
|actually set it on POWER9 hardware. This is done so we don't break
|migration from POWER8 to POWER9, at the cost of silently losing the
|DAWR on the migration.
|For xmon, the 'bd' command will return an error on P9.
|Consequences for users
|For GDB watchpoints (ie 'watch' command) on POWER9 bare metal , GDB
|will accept the command. Unfortunately since there is no hardware
|support for the watchpoint, GDB will software emulate the watchpoint
|making it run very slowly.
|The same will also be true for any guests started on a POWER9
|host. The watchpoint will fail and GDB will fall back to software
|If a guest is started on a POWER8 host, GDB will accept the watchpoint
|and configure the hardware to use the DAWR. This will run at full
|speed since it can use the hardware emulation. Unfortunately if this
|guest is migrated to a POWER9 host, the watchpoint will be lost on the
|POWER9. Loads and stores to the watchpoint locations will not be
|trapped in GDB. The watchpoint is remembered, so if the guest is
|migrated back to the POWER8 host, it will start working again.
|Force enabling the DAWR
|Kernels (since ~v5.2) have an option to force enable the DAWR via::
| echo Y > /sys/kernel/debug/powerpc/dawr_enable_dangerous
|This enables the DAWR even on POWER9.
|This is a dangerous setting, USE AT YOUR OWN RISK.
|Some users may not care about a bad user crashing their box
|(ie. single user/desktop systems) and really want the DAWR. This
|allows them to force enable DAWR.
|This flag can also be used to disable DAWR access. Once this is
|cleared, all DAWR access should be cleared immediately and your
|machine once again safe from crashing.
|Userspace may get confused by toggling this. If DAWR is force
|enabled/disabled between getting the number of breakpoints (via
|PTRACE_GETHWDBGINFO) and setting the breakpoint, userspace will get an
|inconsistent view of what's available. Similarly for guests.
|For the DAWR to be enabled in a KVM guest, the DAWR needs to be force
|enabled in the host AND the guest. For this reason, this won't work on
|POWERVM as it doesn't allow the HCALL to work. Writes of 'Y' to the
|dawr_enable_dangerous file will fail if the hypervisor doesn't support
|writing the DAWR.
|To double check the DAWR is working, run this kernel selftest:
|Any errors/failures/skips mean something is wrong.