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powerpc: Document issues with TM 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>
This commit is contained in:
Michael Neuling 2018-06-25 11:34:56 +10:00 committed by Michael Ellerman
parent c6bef2e9e5
commit aad15ccc4c
1 changed files with 44 additions and 0 deletions
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Documentation/powerpc/transactional_memory.txt
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@ -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.
POWER9
======
TM on POWER9 has issues with storing the complete register state. This
is described in this commit:
commit 4bb3c7a0208fc13ca70598efd109901a7cd45ae7
Author: Paul Mackerras <paulus@ozlabs.org>
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
different ways.
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
POWER9N DD2.2.
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.