1DAWR issues on POWER9
2============================
3
4On POWER9 the DAWR can cause a checkstop if it points to cache
5inhibited (CI) memory. Currently Linux has no way to disinguish CI
6memory when configuring the DAWR, so (for now) the DAWR is disabled by
7this commit:
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
14Technical Details:
15============================
16
17DAWR has 6 different ways of being set.
181) ptrace
192) h_set_mode(DAWR)
203) h_set_dabr()
214) kvmppc_set_one_reg()
225) xmon
23
24For ptrace, we now advertise zero breakpoints on POWER9 via the
25PPC_PTRACE_GETHWDBGINFO call. This results in GDB falling back to
26software emulation of the watchpoint (which is slow).
27
28h_set_mode(DAWR) and h_set_dabr() will now return an error to the
29guest on a POWER9 host. Current Linux guests ignore this error, so
30they will silently not get the DAWR.
31
32kvmppc_set_one_reg() will store the value in the vcpu but won't
33actually set it on POWER9 hardware. This is done so we don't break
34migration from POWER8 to POWER9, at the cost of silently losing the
35DAWR on the migration.
36
37For xmon, the 'bd' command will return an error on P9.
38
39Consequences for users
40============================
41
42For GDB watchpoints (ie 'watch' command) on POWER9 bare metal , GDB
43will accept the command. Unfortunately since there is no hardware
44support for the watchpoint, GDB will software emulate the watchpoint
45making it run very slowly.
46
47The same will also be true for any guests started on a POWER9
48host. The watchpoint will fail and GDB will fall back to software
49emulation.
50
51If a guest is started on a POWER8 host, GDB will accept the watchpoint
52and configure the hardware to use the DAWR. This will run at full
53speed since it can use the hardware emulation. Unfortunately if this
54guest is migrated to a POWER9 host, the watchpoint will be lost on the
55POWER9. Loads and stores to the watchpoint locations will not be
56trapped in GDB. The watchpoint is remembered, so if the guest is
57migrated back to the POWER8 host, it will start working again.
58
59