| Michael Neuling | c6bef2e | 2018-06-25 11:34:55 +1000 | [diff] [blame] | 1 | DAWR issues on POWER9 |
| 2 | ============================ |
| 3 | |
| 4 | On POWER9 the DAWR can cause a checkstop if it points to cache |
| 5 | inhibited (CI) memory. Currently Linux has no way to disinguish CI |
| 6 | memory when configuring the DAWR, so (for now) the DAWR is disabled by |
| 7 | this commit: |
| 8 | |
| 9 | commit 9654153158d3e0684a1bdb76dbababdb7111d5a0 |
| 10 | Author: Michael Neuling <mikey@neuling.org> |
| 11 | Date: Tue Mar 27 15:37:24 2018 +1100 |
| 12 | powerpc: Disable DAWR in the base POWER9 CPU features |
| 13 | |
| 14 | Technical Details: |
| 15 | ============================ |
| 16 | |
| 17 | DAWR has 6 different ways of being set. |
| 18 | 1) ptrace |
| 19 | 2) h_set_mode(DAWR) |
| 20 | 3) h_set_dabr() |
| 21 | 4) kvmppc_set_one_reg() |
| 22 | 5) xmon |
| 23 | |
| 24 | For ptrace, we now advertise zero breakpoints on POWER9 via the |
| 25 | PPC_PTRACE_GETHWDBGINFO call. This results in GDB falling back to |
| 26 | software emulation of the watchpoint (which is slow). |
| 27 | |
| 28 | h_set_mode(DAWR) and h_set_dabr() will now return an error to the |
| 29 | guest on a POWER9 host. Current Linux guests ignore this error, so |
| 30 | they will silently not get the DAWR. |
| 31 | |
| 32 | kvmppc_set_one_reg() will store the value in the vcpu but won't |
| 33 | actually set it on POWER9 hardware. This is done so we don't break |
| 34 | migration from POWER8 to POWER9, at the cost of silently losing the |
| 35 | DAWR on the migration. |
| 36 | |
| 37 | For xmon, the 'bd' command will return an error on P9. |
| 38 | |
| 39 | Consequences for users |
| 40 | ============================ |
| 41 | |
| 42 | For GDB watchpoints (ie 'watch' command) on POWER9 bare metal , GDB |
| 43 | will accept the command. Unfortunately since there is no hardware |
| 44 | support for the watchpoint, GDB will software emulate the watchpoint |
| 45 | making it run very slowly. |
| 46 | |
| 47 | The same will also be true for any guests started on a POWER9 |
| 48 | host. The watchpoint will fail and GDB will fall back to software |
| 49 | emulation. |
| 50 | |
| 51 | If a guest is started on a POWER8 host, GDB will accept the watchpoint |
| 52 | and configure the hardware to use the DAWR. This will run at full |
| 53 | speed since it can use the hardware emulation. Unfortunately if this |
| 54 | guest is migrated to a POWER9 host, the watchpoint will be lost on the |
| 55 | POWER9. Loads and stores to the watchpoint locations will not be |
| 56 | trapped in GDB. The watchpoint is remembered, so if the guest is |
| 57 | migrated back to the POWER8 host, it will start working again. |
| 58 | |