Commit | Line | Data |
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4d2e26a3 | 1 | ===================== |
c6bef2e9 | 2 | DAWR issues on POWER9 |
4d2e26a3 | 3 | ===================== |
c6bef2e9 | 4 | |
29b861ea JS |
5 | On POWER9 the Data Address Watchpoint Register (DAWR) can cause a checkstop |
6 | if it points to cache inhibited (CI) memory. Currently Linux has no way to | |
7 | disinguish CI memory when configuring the DAWR, so (for now) the DAWR is | |
4d2e26a3 | 8 | disabled by this commit:: |
c6bef2e9 MN |
9 | |
10 | commit 9654153158d3e0684a1bdb76dbababdb7111d5a0 | |
11 | Author: Michael Neuling <mikey@neuling.org> | |
12 | Date: Tue Mar 27 15:37:24 2018 +1100 | |
13 | powerpc: Disable DAWR in the base POWER9 CPU features | |
14 | ||
15 | Technical Details: | |
4d2e26a3 | 16 | ================== |
c6bef2e9 MN |
17 | |
18 | DAWR has 6 different ways of being set. | |
19 | 1) ptrace | |
20 | 2) h_set_mode(DAWR) | |
21 | 3) h_set_dabr() | |
22 | 4) kvmppc_set_one_reg() | |
23 | 5) xmon | |
24 | ||
25 | For ptrace, we now advertise zero breakpoints on POWER9 via the | |
26 | PPC_PTRACE_GETHWDBGINFO call. This results in GDB falling back to | |
27 | software emulation of the watchpoint (which is slow). | |
28 | ||
29 | h_set_mode(DAWR) and h_set_dabr() will now return an error to the | |
30 | guest on a POWER9 host. Current Linux guests ignore this error, so | |
31 | they will silently not get the DAWR. | |
32 | ||
33 | kvmppc_set_one_reg() will store the value in the vcpu but won't | |
34 | actually set it on POWER9 hardware. This is done so we don't break | |
35 | migration from POWER8 to POWER9, at the cost of silently losing the | |
36 | DAWR on the migration. | |
37 | ||
38 | For xmon, the 'bd' command will return an error on P9. | |
39 | ||
40 | Consequences for users | |
4d2e26a3 | 41 | ====================== |
c6bef2e9 MN |
42 | |
43 | For GDB watchpoints (ie 'watch' command) on POWER9 bare metal , GDB | |
44 | will accept the command. Unfortunately since there is no hardware | |
45 | support for the watchpoint, GDB will software emulate the watchpoint | |
46 | making it run very slowly. | |
47 | ||
48 | The same will also be true for any guests started on a POWER9 | |
49 | host. The watchpoint will fail and GDB will fall back to software | |
50 | emulation. | |
51 | ||
52 | If a guest is started on a POWER8 host, GDB will accept the watchpoint | |
53 | and configure the hardware to use the DAWR. This will run at full | |
54 | speed since it can use the hardware emulation. Unfortunately if this | |
55 | guest is migrated to a POWER9 host, the watchpoint will be lost on the | |
56 | POWER9. Loads and stores to the watchpoint locations will not be | |
57 | trapped in GDB. The watchpoint is remembered, so if the guest is | |
58 | migrated back to the POWER8 host, it will start working again. | |
59 | ||
c1fe190c | 60 | Force enabling the DAWR |
4d2e26a3 MCC |
61 | ======================= |
62 | Kernels (since ~v5.2) have an option to force enable the DAWR via:: | |
c1fe190c MN |
63 | |
64 | echo Y > /sys/kernel/debug/powerpc/dawr_enable_dangerous | |
65 | ||
66 | This enables the DAWR even on POWER9. | |
67 | ||
68 | This is a dangerous setting, USE AT YOUR OWN RISK. | |
69 | ||
70 | Some users may not care about a bad user crashing their box | |
71 | (ie. single user/desktop systems) and really want the DAWR. This | |
72 | allows them to force enable DAWR. | |
73 | ||
74 | This flag can also be used to disable DAWR access. Once this is | |
75 | cleared, all DAWR access should be cleared immediately and your | |
76 | machine once again safe from crashing. | |
77 | ||
78 | Userspace may get confused by toggling this. If DAWR is force | |
79 | enabled/disabled between getting the number of breakpoints (via | |
80 | PTRACE_GETHWDBGINFO) and setting the breakpoint, userspace will get an | |
81 | inconsistent view of what's available. Similarly for guests. | |
82 | ||
83 | For the DAWR to be enabled in a KVM guest, the DAWR needs to be force | |
84 | enabled in the host AND the guest. For this reason, this won't work on | |
85 | POWERVM as it doesn't allow the HCALL to work. Writes of 'Y' to the | |
86 | dawr_enable_dangerous file will fail if the hypervisor doesn't support | |
87 | writing the DAWR. | |
88 | ||
89 | To double check the DAWR is working, run this kernel selftest: | |
4d2e26a3 | 90 | |
c1fe190c | 91 | tools/testing/selftests/powerpc/ptrace/ptrace-hwbreak.c |
4d2e26a3 | 92 | |
c1fe190c | 93 | Any errors/failures/skips mean something is wrong. |