[PATCH v2 1/2] powerpc: Document issues with the DAWR on POWER9
Signed-off-by: Michael Neuling <[hidden email]>
Acked-by: Stewart Smith <[hidden email]>
Spelling mistakes :-/
Documentation/powerpc/DAWR-POWER9.txt | 58 +++++++++++++++++++++++++++
1 file changed, 58 insertions(+)
create mode 100644 Documentation/powerpc/DAWR-POWER9.txt
diff --git a/Documentation/powerpc/DAWR-POWER9.txt b/Documentation/powerpc/DAWR-POWER9.txt
new file mode 100644
@@ -0,0 +1,58 @@
+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
+ commit 9654153158d3e0684a1bdb76dbababdb7111d5a0
+ Author: Michael Neuling <[hidden email]>
+ Date: Tue Mar 27 15:37:24 2018 +1100
+ powerpc: Disable DAWR in the base POWER9 CPU features
+DAWR has 6 different ways of being set.
+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
+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.
diff --git a/Documentation/powerpc/transactional_memory.txt b/Documentation/powerpc/transactional_memory.txt
index e32fdbb4c9..52c023e14f 100644
@@ -198,3 +198,47 @@ presented). The transaction cannot then be continued and will take the failure
handler route. Furthermore, the transactional 2nd register state will be
inaccessible. GDB can currently be used on programs using TM, but not sensibly
in parts within transactions.
+TM on POWER9 has issues with storing the complete register state. This
+is described in this commit:
+ commit 4bb3c7a0208fc13ca70598efd109901a7cd45ae7
+ Author: Paul Mackerras <[hidden email]>
+ Date: Wed Mar 21 21:32:01 2018 +1100
+ KVM: PPC: Book3S HV: Work around transactional memory bugs in POWER9
+To account for this different POWER9 chips have TM enabled in
+On POWER9N DD2.01 and below, TM is disabled. ie
+HWCAP2[PPC_FEATURE2_HTM] is not set.
+On POWER9N DD2.1 TM is configured by firmware to always abort a
+transaction when tm suspend occurs. So tsuspend will cause a
+transaction to be aborted and rolled back. Kernel exceptions will also
+cause the transaction to be aborted and rolled back and the exception
+will not occur. If userspace constructs a sigcontext that enables TM
+suspend, the sigcontext will be rejected by the kernel. This mode is
+advertised to users with HWCAP2[PPC_FEATURE2_HTM_NO_SUSPEND] set.
+HWCAP2[PPC_FEATURE2_HTM] is not set in this mode.
+On POWER9N DD2.2 and above, KVM and POWERVM emulate TM for guests (as
+described in commit 4bb3c7a0208f), hence TM is enabled for guests
+ie. HWCAP2[PPC_FEATURE2_HTM] is set for guest userspace. Guests that
+makes heavy use of TM suspend (tsuspend or kernel suspend) will result
+in traps into the hypervisor and hence will suffer a performance
+degradation. Host userspace has TM disabled
+ie. HWCAP2[PPC_FEATURE2_HTM] is not set. (although we make enable it
+at some point in the future if we bring the emulation into host
+userspace context switching).
+POWER9C DD1.2 and above are only available with POWERVM and hence
+Linux only runs as a guest. On these systems TM is emulated like on
+Guest migration from POWER8 to POWER9 will work with POWER9N DD2.2 and
+POWER9C DD1.2. Since earlier POWER9 processors don't support TM
+emulation, migration from POWER8 to POWER9 is not supported there.