diff options
Diffstat (limited to 'Documentation/powerpc/DAWR-POWER9.txt')
| -rw-r--r-- | Documentation/powerpc/DAWR-POWER9.txt | 90 |
1 files changed, 0 insertions, 90 deletions
diff --git a/Documentation/powerpc/DAWR-POWER9.txt b/Documentation/powerpc/DAWR-POWER9.txt deleted file mode 100644 index ecdbb076438c..000000000000 --- a/Documentation/powerpc/DAWR-POWER9.txt +++ /dev/null @@ -1,90 +0,0 @@ -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 -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> - Date: Tue Mar 27 15:37:24 2018 +1100 - powerpc: Disable DAWR in the base POWER9 CPU features - -Technical Details: -============================ - -DAWR has 6 different ways of being set. -1) ptrace -2) h_set_mode(DAWR) -3) h_set_dabr() -4) kvmppc_set_one_reg() -5) xmon - -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 -emulation. - -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: - tools/testing/selftests/powerpc/ptrace/ptrace-hwbreak.c -Any errors/failures/skips mean something is wrong. |