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| author | Michael Neuling <mikey@neuling.org> | 2019-04-01 17:03:12 +1100 |
|---|---|---|
| committer | Michael Ellerman <mpe@ellerman.id.au> | 2019-04-20 22:20:45 +1000 |
| commit | c1fe190c06723322f2dfac31d3b982c581e434ef (patch) | |
| tree | b6d101f9d5fc11436af222ff574f3010f19ae340 /Documentation/powerpc | |
| parent | 88ec6b93c8e7d6d4ffaf6ad6395ceb3bf552de15 (diff) | |
| download | lwn-c1fe190c06723322f2dfac31d3b982c581e434ef.tar.gz lwn-c1fe190c06723322f2dfac31d3b982c581e434ef.zip | |
powerpc: Add force enable of DAWR on P9 option
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@neuling.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Diffstat (limited to 'Documentation/powerpc')
| -rw-r--r-- | Documentation/powerpc/DAWR-POWER9.txt | 32 |
1 files changed, 32 insertions, 0 deletions
diff --git a/Documentation/powerpc/DAWR-POWER9.txt b/Documentation/powerpc/DAWR-POWER9.txt index 2feaa6619658..bdec03650941 100644 --- a/Documentation/powerpc/DAWR-POWER9.txt +++ b/Documentation/powerpc/DAWR-POWER9.txt @@ -56,3 +56,35 @@ 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: + tools/testing/selftests/powerpc/ptrace/ptrace-hwbreak.c +Any errors/failures/skips mean something is wrong. |