[PATCH v2] powerpc: Add force enable of DAWR on P9 option

[Michael Ellerman]

On Mon, 2019-04-01 at 06:03:12 UTC, Michael Neuling wrote:
> This adds a flag so that the DAWR can be enabled on P9 via:
>   echo Y > /sys/kernel/debug/powerpc/dawr_enable_dangerous
> 
> The DAWR was previously force disabled on POWER9 in:
>   9654153158 powerpc: Disable DAWR in the base POWER9 CPU features
> Also see Documentation/powerpc/DAWR-POWER9.txt
> 
> 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:
>   tools/testing/selftests/powerpc/ptrace/ptrace-hwbreak.c
> Any errors/failures/skips mean something is wrong.
> 
> Signed-off-by: Michael Neuling <mikey at neuling.org>

Applied to powerpc topic/ppc-kvm, thanks.

https://git.kernel.org/powerpc/c/c1fe190c06723322f2dfac31d3b982c5

cheers