1 =====================
2 DAWR issues on POWER9
3 =====================
4
5 On older POWER9 processors, the Data Address W
6 cause a checkstop if it points to cache inhibi
7 has no way to distinguish CI memory when confi
8 systems, the DAWR is disabled.
9
10 Affected processor revisions
11 ============================
12
13 This issue is only present on processors prior
14 found in /proc/cpuinfo::
15
16 processor : 0
17 cpu : POWER9, altivec supporte
18 clock : 3800.000000MHz
19 revision : 2.3 (pvr 004e 1203)
20
21 On a system with the issue, the DAWR is disabl
22
23 Technical Details:
24 ==================
25
26 DAWR has 6 different ways of being set.
27 1) ptrace
28 2) h_set_mode(DAWR)
29 3) h_set_dabr()
30 4) kvmppc_set_one_reg()
31 5) xmon
32
33 For ptrace, we now advertise zero breakpoints
34 PPC_PTRACE_GETHWDBGINFO call. This results in
35 software emulation of the watchpoint (which is
36
37 h_set_mode(DAWR) and h_set_dabr() will now ret
38 guest on a POWER9 host. Current Linux guests i
39 they will silently not get the DAWR.
40
41 kvmppc_set_one_reg() will store the value in t
42 actually set it on POWER9 hardware. This is do
43 migration from POWER8 to POWER9, at the cost o
44 DAWR on the migration.
45
46 For xmon, the 'bd' command will return an erro
47
48 Consequences for users
49 ======================
50
51 For GDB watchpoints (ie 'watch' command) on PO
52 will accept the command. Unfortunately since t
53 support for the watchpoint, GDB will software
54 making it run very slowly.
55
56 The same will also be true for any guests star
57 host. The watchpoint will fail and GDB will fa
58 emulation.
59
60 If a guest is started on a POWER8 host, GDB wi
61 and configure the hardware to use the DAWR. Th
62 speed since it can use the hardware emulation.
63 guest is migrated to a POWER9 host, the watchp
64 POWER9. Loads and stores to the watchpoint loc
65 trapped in GDB. The watchpoint is remembered,
66 migrated back to the POWER8 host, it will star
67
68 Force enabling the DAWR
69 =======================
70 Kernels (since ~v5.2) have an option to force
71
72 echo Y > /sys/kernel/debug/powerpc/dawr_enab
73
74 This enables the DAWR even on POWER9.
75
76 This is a dangerous setting, USE AT YOUR OWN R
77
78 Some users may not care about a bad user crash
79 (ie. single user/desktop systems) and really w
80 allows them to force enable DAWR.
81
82 This flag can also be used to disable DAWR acc
83 cleared, all DAWR access should be cleared imm
84 machine once again safe from crashing.
85
86 Userspace may get confused by toggling this. I
87 enabled/disabled between getting the number of
88 PTRACE_GETHWDBGINFO) and setting the breakpoin
89 inconsistent view of what's available. Similar
90
91 For the DAWR to be enabled in a KVM guest, the
92 enabled in the host AND the guest. For this re
93 POWERVM as it doesn't allow the HCALL to work.
94 dawr_enable_dangerous file will fail if the hy
95 writing the DAWR.
96
97 To double check the DAWR is working, run this
98
99 tools/testing/selftests/powerpc/ptrace/ptrac
100
101 Any errors/failures/skips mean something is wr
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