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+=====================
+DAWR issues on POWER9
+=====================
+
+On older POWER9 processors, the Data Address Watchpoint Register (DAWR) can
+cause a checkstop if it points to cache inhibited (CI) memory. Currently Linux
+has no way to distinguish CI memory when configuring the DAWR, so on affected
+systems, the DAWR is disabled.
+
+Affected processor revisions
+============================
+
+This issue is only present on processors prior to v2.3. The revision can be
+found in /proc/cpuinfo::
+
+    processor       : 0
+    cpu             : POWER9, altivec supported
+    clock           : 3800.000000MHz
+    revision        : 2.3 (pvr 004e 1203)
+
+On a system with the issue, the DAWR is disabled as detailed below.
+
+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.
+
+Force enabling the DAWR
+=======================
+Kernels (since ~v5.2) have an option to force enable the DAWR via::
+
+  echo Y > /sys/kernel/debug/powerpc/dawr_enable_dangerous
+
+This enables the DAWR even on POWER9.
+
+This is a dangerous setting, USE AT YOUR OWN RISK.
+
+Some users may not care about a bad user crashing their box
+(ie. single user/desktop systems) and really want the DAWR.  This
+allows them to force enable DAWR.
+
+This flag can also be used to disable DAWR access. Once this is
+cleared, all DAWR access should be cleared immediately and your
+machine once again safe from crashing.
+
+Userspace may get confused by toggling this. If DAWR is force
+enabled/disabled between getting the number of breakpoints (via
+PTRACE_GETHWDBGINFO) and setting the breakpoint, userspace will get an
+inconsistent view of what's available. Similarly for guests.
+
+For the DAWR to be enabled in a KVM guest, the DAWR needs to be force
+enabled in the host AND the guest. For this reason, this won't work on
+POWERVM as it doesn't allow the HCALL to work. Writes of 'Y' to the
+dawr_enable_dangerous file will fail if the hypervisor doesn't support
+writing the DAWR.
+
+To double check the DAWR is working, run this kernel selftest:
+
+  tools/testing/selftests/powerpc/ptrace/ptrace-hwbreak.c
+
+Any errors/failures/skips mean something is wrong.