DAWR issues on POWER9

============================

On POWER9 the DAWR can cause a checkstop if it points to cache

inhibited (CI) memory. Currently Linux has no way to disinguish CI

memory when configuring the DAWR, so (for now) the DAWR is disabled by

this commit:

commit 9654153158d3e0684a1bdb76dbababdb7111d5a0

Author: Michael Neuling <mikey@neuling.org>

Date: Tue Mar 27 15:37:24 2018 +1100

powerpc: Disable DAWR in the base POWER9 CPU features

Technical Details:

============================

DAWR has 6 different ways of being set.

1) ptrace

2) h_set_mode(DAWR)

3) h_set_dabr()

4) kvmppc_set_one_reg()

5) xmon

For ptrace, we now advertise zero breakpoints on POWER9 via the

PPC_PTRACE_GETHWDBGINFO call. This results in GDB falling back to

software emulation of the watchpoint (which is slow).

h_set_mode(DAWR) and h_set_dabr() will now return an error to the

guest on a POWER9 host. Current Linux guests ignore this error, so

they will silently not get the DAWR.

kvmppc_set_one_reg() will store the value in the vcpu but won't

actually set it on POWER9 hardware. This is done so we don't break

migration from POWER8 to POWER9, at the cost of silently losing the

DAWR on the migration.

For xmon, the 'bd' command will return an error on P9.

Consequences for users

============================

For GDB watchpoints (ie 'watch' command) on POWER9 bare metal , GDB

will accept the command. Unfortunately since there is no hardware

support for the watchpoint, GDB will software emulate the watchpoint

making it run very slowly.

The same will also be true for any guests started on a POWER9

host. The watchpoint will fail and GDB will fall back to software

emulation.

If a guest is started on a POWER8 host, GDB will accept the watchpoint

and configure the hardware to use the DAWR. This will run at full

speed since it can use the hardware emulation. Unfortunately if this

guest is migrated to a POWER9 host, the watchpoint will be lost on the

POWER9. Loads and stores to the watchpoint locations will not be

trapped in GDB. The watchpoint is remembered, so if the guest is

migrated back to the POWER8 host, it will start working again.