1=====================
2DAWR issues on POWER9
3=====================
4
5On POWER9 the Data Address Watchpoint Register (DAWR) can cause a checkstop
6if it points to cache inhibited (CI) memory. Currently Linux has no way to
7disinguish CI memory when configuring the DAWR, so (for now) the DAWR is
8disabled by this commit::
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
15Technical Details:
16==================
17
18DAWR has 6 different ways of being set.
191) ptrace
202) h_set_mode(DAWR)
213) h_set_dabr()
224) kvmppc_set_one_reg()
235) xmon
24
25For ptrace, we now advertise zero breakpoints on POWER9 via the
26PPC_PTRACE_GETHWDBGINFO call. This results in GDB falling back to
27software emulation of the watchpoint (which is slow).
28
29h_set_mode(DAWR) and h_set_dabr() will now return an error to the
30guest on a POWER9 host. Current Linux guests ignore this error, so
31they will silently not get the DAWR.
32
33kvmppc_set_one_reg() will store the value in the vcpu but won't
34actually set it on POWER9 hardware. This is done so we don't break
35migration from POWER8 to POWER9, at the cost of silently losing the
36DAWR on the migration.
37
38For xmon, the 'bd' command will return an error on P9.
39
40Consequences for users
41======================
42
43For GDB watchpoints (ie 'watch' command) on POWER9 bare metal , GDB
44will accept the command. Unfortunately since there is no hardware
45support for the watchpoint, GDB will software emulate the watchpoint
46making it run very slowly.
47
48The same will also be true for any guests started on a POWER9
49host. The watchpoint will fail and GDB will fall back to software
50emulation.
51
52If a guest is started on a POWER8 host, GDB will accept the watchpoint
53and configure the hardware to use the DAWR. This will run at full
54speed since it can use the hardware emulation. Unfortunately if this
55guest is migrated to a POWER9 host, the watchpoint will be lost on the
56POWER9. Loads and stores to the watchpoint locations will not be
57trapped in GDB. The watchpoint is remembered, so if the guest is
58migrated back to the POWER8 host, it will start working again.
59
60Force enabling the DAWR
61=======================
62Kernels (since ~v5.2) have an option to force enable the DAWR via::
63
64  echo Y > /sys/kernel/debug/powerpc/dawr_enable_dangerous
65
66This enables the DAWR even on POWER9.
67
68This is a dangerous setting, USE AT YOUR OWN RISK.
69
70Some users may not care about a bad user crashing their box
71(ie. single user/desktop systems) and really want the DAWR.  This
72allows them to force enable DAWR.
73
74This flag can also be used to disable DAWR access. Once this is
75cleared, all DAWR access should be cleared immediately and your
76machine once again safe from crashing.
77
78Userspace may get confused by toggling this. If DAWR is force
79enabled/disabled between getting the number of breakpoints (via
80PTRACE_GETHWDBGINFO) and setting the breakpoint, userspace will get an
81inconsistent view of what's available. Similarly for guests.
82
83For the DAWR to be enabled in a KVM guest, the DAWR needs to be force
84enabled in the host AND the guest. For this reason, this won't work on
85POWERVM as it doesn't allow the HCALL to work. Writes of 'Y' to the
86dawr_enable_dangerous file will fail if the hypervisor doesn't support
87writing the DAWR.
88
89To double check the DAWR is working, run this kernel selftest:
90
91  tools/testing/selftests/powerpc/ptrace/ptrace-hwbreak.c
92
93Any errors/failures/skips mean something is wrong.
94