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Commit | Line | Data |
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c6bef2e9 MN |
1 | DAWR issues on POWER9 |
2 | ============================ | |
3 | ||
29b861ea JS |
4 | On POWER9 the Data Address Watchpoint Register (DAWR) can cause a checkstop |
5 | if it points to cache inhibited (CI) memory. Currently Linux has no way to | |
6 | disinguish CI memory when configuring the DAWR, so (for now) the DAWR is | |
7 | disabled by this commit: | |
c6bef2e9 MN |
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 | ||
c1fe190c MN |
59 | Force enabling the DAWR |
60 | ============================= | |
61 | Kernels (since ~v5.2) have an option to force enable the DAWR via: | |
62 | ||
63 | echo Y > /sys/kernel/debug/powerpc/dawr_enable_dangerous | |
64 | ||
65 | This enables the DAWR even on POWER9. | |
66 | ||
67 | This is a dangerous setting, USE AT YOUR OWN RISK. | |
68 | ||
69 | Some users may not care about a bad user crashing their box | |
70 | (ie. single user/desktop systems) and really want the DAWR. This | |
71 | allows them to force enable DAWR. | |
72 | ||
73 | This flag can also be used to disable DAWR access. Once this is | |
74 | cleared, all DAWR access should be cleared immediately and your | |
75 | machine once again safe from crashing. | |
76 | ||
77 | Userspace may get confused by toggling this. If DAWR is force | |
78 | enabled/disabled between getting the number of breakpoints (via | |
79 | PTRACE_GETHWDBGINFO) and setting the breakpoint, userspace will get an | |
80 | inconsistent view of what's available. Similarly for guests. | |
81 | ||
82 | For the DAWR to be enabled in a KVM guest, the DAWR needs to be force | |
83 | enabled in the host AND the guest. For this reason, this won't work on | |
84 | POWERVM as it doesn't allow the HCALL to work. Writes of 'Y' to the | |
85 | dawr_enable_dangerous file will fail if the hypervisor doesn't support | |
86 | writing the DAWR. | |
87 | ||
88 | To double check the DAWR is working, run this kernel selftest: | |
89 | tools/testing/selftests/powerpc/ptrace/ptrace-hwbreak.c | |
90 | Any errors/failures/skips mean something is wrong. |