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Diffstat (limited to 'Documentation/powerpc/DAWR-POWER9.txt')
| -rw-r--r-- | Documentation/powerpc/DAWR-POWER9.txt | 40 |
1 files changed, 36 insertions, 4 deletions
diff --git a/Documentation/powerpc/DAWR-POWER9.txt b/Documentation/powerpc/DAWR-POWER9.txt index 2feaa6619658..ecdbb076438c 100644 --- a/Documentation/powerpc/DAWR-POWER9.txt +++ b/Documentation/powerpc/DAWR-POWER9.txt @@ -1,10 +1,10 @@ DAWR issues on POWER9 ============================ -On POWER9 the DAWR can cause a checkstop if it points to cache -inhibited (CI) memory. Currently Linux has no way to disinguish CI -memory when configuring the DAWR, so (for now) the DAWR is disabled by -this commit: +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 +disinguish CI memory when configuring the DAWR, so (for now) the DAWR is +disabled by this commit: commit 9654153158d3e0684a1bdb76dbababdb7111d5a0 Author: Michael Neuling <mikey@neuling.org> @@ -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. |