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| -rw-r--r-- | Documentation/powerpc/DAWR-POWER9.txt | 58 |
1 files changed, 58 insertions, 0 deletions
diff --git a/Documentation/powerpc/DAWR-POWER9.txt b/Documentation/powerpc/DAWR-POWER9.txt new file mode 100644 index 000000000..2feaa6619 --- /dev/null +++ b/Documentation/powerpc/DAWR-POWER9.txt @@ -0,0 +1,58 @@ +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: + + 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. + |