| 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. |
| |
