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Date: Tue, 16 Mar 2021 17:36:04 +1000
From: Nicholas Piggin <npiggin@gmail.com>
Subject: Re: [PATCH v3 28/32] powerpc/64s: interrupt implement exit logic in C
To: Christophe Leroy <christophe.leroy@csgroup.eu>,
 linuxppc-dev@lists.ozlabs.org
References: <20200225173541.1549955-1-npiggin@gmail.com>
 <20200225173541.1549955-29-npiggin@gmail.com>
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Cc: Michal Suchanek <msuchanek@suse.de>
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Excerpts from Christophe Leroy's message of March 15, 2021 11:41 pm:
>=20
>=20
> Le 25/02/2020 =C3=A0 18:35, Nicholas Piggin a =C3=A9crit=C2=A0:
>> Implement the bulk of interrupt return logic in C. The asm return code
>> must handle a few cases: restoring full GPRs, and emulating stack store.
>>=20
>> The stack store emulation is significantly simplfied, rather than creati=
ng
>> a new return frame and switching to that before performing the store, it
>> uses the PACA to keep a scratch register around to perform thestore.
>>=20
>> The asm return code is moved into 64e for now. The new logic has made
>> allowance for 64e, but I don't have a full environment that works well
>> to test it, and even booting in emulated qemu is not great for stress
>> testing. 64e shouldn't be too far off working with this, given a bit
>> more testing and auditing of the logic.
>>=20
>> This is slightly faster on a POWER9 (page fault speed increases about
>> 1.1%), probably due to reduced mtmsrd.
>>=20
>> Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
>> Signed-off-by: Michal Suchanek <msuchanek@suse.de>
>> ---
>=20
> ...
>=20
>> +notrace unsigned long interrupt_exit_user_prepare(struct pt_regs *regs,=
 unsigned long msr)
>> +{
>=20
> ...
>=20
>> +
>> +#ifdef CONFIG_PPC_TRANSACTIONAL_MEM
>> +	local_paca->tm_scratch =3D regs->msr;
>> +#endif
>=20
> Could we define a helper for that in asm/tm.h, that voids when CONFIG_PPC=
_TRANSACTIONAL_MEM is not=20
> selected ?

Yeah I wanted to do something about that. I don't know what it's used=20
for here. I guess it saves the return MSR so if that causes a crash then=20
the next oops would see it, but I wonder if we can just get that from=20
SRR1 + program check error codes, or if there is something we can't
reconstruct from there. Have to check with someone who knows TM better.

Thanks,
Nick