| 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. | |