1 ===================== 2 DAWR issues on POWER9 3 ===================== 4 5 On older POWER9 processors, the Data Address Watchpoint Register (DAWR) can 6 cause a checkstop if it points to cache inhibited (CI) memory. Currently Linux 7 has no way to distinguish CI memory when configuring the DAWR, so on affected 8 systems, the DAWR is disabled. 9 10 Affected processor revisions 11 ============================ 12 13 This issue is only present on processors prior to v2.3. The revision can be 14 found in /proc/cpuinfo:: 15 16 processor : 0 17 cpu : POWER9, altivec supported 18 clock : 3800.000000MHz 19 revision : 2.3 (pvr 004e 1203) 20 21 On a system with the issue, the DAWR is disabled as detailed below. 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 on POWER9 via the 34 PPC_PTRACE_GETHWDBGINFO call. This results in GDB falling back to 35 software emulation of the watchpoint (which is slow). 36 37 h_set_mode(DAWR) and h_set_dabr() will now return an error to the 38 guest on a POWER9 host. Current Linux guests ignore this error, so 39 they will silently not get the DAWR. 40 41 kvmppc_set_one_reg() will store the value in the vcpu but won't 42 actually set it on POWER9 hardware. This is done so we don't break 43 migration from POWER8 to POWER9, at the cost of silently losing the 44 DAWR on the migration. 45 46 For xmon, the 'bd' command will return an error on P9. 47 48 Consequences for users 49 ====================== 50 51 For GDB watchpoints (ie 'watch' command) on POWER9 bare metal , GDB 52 will accept the command. Unfortunately since there is no hardware 53 support for the watchpoint, GDB will software emulate the watchpoint 54 making it run very slowly. 55 56 The same will also be true for any guests started on a POWER9 57 host. The watchpoint will fail and GDB will fall back to software 58 emulation. 59 60 If a guest is started on a POWER8 host, GDB will accept the watchpoint 61 and configure the hardware to use the DAWR. This will run at full 62 speed since it can use the hardware emulation. Unfortunately if this 63 guest is migrated to a POWER9 host, the watchpoint will be lost on the 64 POWER9. Loads and stores to the watchpoint locations will not be 65 trapped in GDB. The watchpoint is remembered, so if the guest is 66 migrated back to the POWER8 host, it will start working again. 67 68 Force enabling the DAWR 69 ======================= 70 Kernels (since ~v5.2) have an option to force enable the DAWR via:: 71 72 echo Y > /sys/kernel/debug/powerpc/dawr_enable_dangerous 73 74 This enables the DAWR even on POWER9. 75 76 This is a dangerous setting, USE AT YOUR OWN RISK. 77 78 Some users may not care about a bad user crashing their box 79 (ie. single user/desktop systems) and really want the DAWR. This 80 allows them to force enable DAWR. 81 82 This flag can also be used to disable DAWR access. Once this is 83 cleared, all DAWR access should be cleared immediately and your 84 machine once again safe from crashing. 85 86 Userspace may get confused by toggling this. If DAWR is force 87 enabled/disabled between getting the number of breakpoints (via 88 PTRACE_GETHWDBGINFO) and setting the breakpoint, userspace will get an 89 inconsistent view of what's available. Similarly for guests. 90 91 For the DAWR to be enabled in a KVM guest, the DAWR needs to be force 92 enabled in the host AND the guest. For this reason, this won't work on 93 POWERVM as it doesn't allow the HCALL to work. Writes of 'Y' to the 94 dawr_enable_dangerous file will fail if the hypervisor doesn't support 95 writing the DAWR. 96 97 To double check the DAWR is working, run this kernel selftest: 98 99 tools/testing/selftests/powerpc/ptrace/ptrace-hwbreak.c 100 101 Any errors/failures/skips mean something is wrong.
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