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powerpc: Document issues with the DAWR on POWER9 

Signed-off-by: Michael Neuling <mikey@neuling.org>
Acked-by: Stewart Smith <stewart@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
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Michael Neuling 2018-06-25 11:34:55 +10:00 committed by Michael Ellerman
parent 3bfb450ee7
commit c6bef2e9e5
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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.