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powerpc/tm: Document h/rfid and mtmsrd quirk

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The ISA has a quirk that's useful for the Linux implementation.
Document it here so others are less likely to trip over it.

Suggested-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200325040546.3091563-1-mikey@neuling.org
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Michael Neuling 2020-03-25 15:05:46 +11:00 committed by Michael Ellerman
parent 91ffeaa7e5
commit b8707e2374
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Documentation/powerpc/transactional_memory.rst
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@ -245,3 +245,30 @@ 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.
Kernel implementation
=====================
h/rfid mtmsrd quirk
-------------------
As defined in the ISA, rfid has a quirk which is useful in early
exception handling. When in a userspace transaction and we enter the
kernel via some exception, MSR will end up as TM=0 and TS=01 (ie. TM
off but TM suspended). Regularly the kernel will want change bits in
the MSR and will perform an rfid to do this. In this case rfid can
have SRR0 TM = 0 and TS = 00 (ie. TM off and non transaction) and the
resulting MSR will retain TM = 0 and TS=01 from before (ie. stay in
suspend). This is a quirk in the architecture as this would normally
be a transition from TS=01 to TS=00 (ie. suspend -> non transactional)
which is an illegal transition.
This quirk is described the architecture in the definition of rfid
with these lines:
if (MSR 29:31 ¬ = 0b010 | SRR1 29:31 ¬ = 0b000) then
MSR 29:31 <- SRR1 29:31
hrfid and mtmsrd have the same quirk.
The Linux kernel uses this quirk in it's early exception handling.