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Date: Mon, 01 Mar 2021 10:47:12 +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>
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 <bc4ec2c0-28e5-4004-d1eb-54058a699af4@csgroup.eu>
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Excerpts from Christophe Leroy's message of February 27, 2021 8:07 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
>=20
> This series, and especially this patch has added a awfull number of BUG_O=
N() traps.
>=20
> We have an issue open at https://github.com/linuxppc/issues/issues/88 sin=
ce 2017 for reducing the=20
> number of BUG_ON()s
>=20
> And the kernel Documentation is explicit on the willingness to deprecate =
BUG_ON(), see=20
> https://www.kernel.org/doc/html/latest/process/deprecated.html?highlight=
=3Dbug_on :
>=20
> BUG() and BUG_ON()
> Use WARN() and WARN_ON() instead, and handle the =E2=80=9Cimpossible=E2=
=80=9D error condition as gracefully as=20
> possible. While the BUG()-family of APIs were originally designed to act =
as an =E2=80=9Cimpossible=20
> situation=E2=80=9D assert and to kill a kernel thread =E2=80=9Csafely=E2=
=80=9D, they turn out to just be too risky. (e.g.=20
> =E2=80=9CIn what order do locks need to be released? Have various states =
been restored?=E2=80=9D) Very commonly,=20
> using BUG() will destabilize a system or entirely break it, which makes i=
t impossible to debug or=20
> even get viable crash reports. Linus has very strong feelings about this.
>=20
> So ... can we do something cleaner with all the BUG_ON()s recently added =
?

Yeah you're right. Some of it is probably overkill due to paranoia when=20
developing the series.

Now we have a bit more confidence we could probably look at cutting down=20
on these.

I do get a bit concerned about detecting a problem in some code like=20
this and attempting to just continue, it usually means the system is=20
going to crash pretty badly anyway (and the WARN_ON trap interrupt is
probably going to finish you off anyway). So I think removing the more
obvious checks entirely (maybe with a PPC DEBUG config option) is the
right way to go.

Thanks,
Nick