linux-kernel - Re: [PATCH v2] powerpc: slightly improve cache helpers

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Message-ID: <875znt7izy.fsf@concordia.ellerman.id.au>
Date:   Tue, 23 Jul 2019 09:21:53 +1000
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Segher Boessenkool <segher@...nel.crashing.org>
Cc:     Nathan Chancellor <natechancellor@...il.com>,
        Christophe Leroy <christophe.leroy@....fr>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
        clang-built-linux@...glegroups.com
Subject: Re: [PATCH v2] powerpc: slightly improve cache helpers

Segher Boessenkool <segher@...nel.crashing.org> writes:
> On Mon, Jul 22, 2019 at 08:15:14PM +1000, Michael Ellerman wrote:
>> Segher Boessenkool <segher@...nel.crashing.org> writes:
>> > On Sun, Jul 21, 2019 at 12:58:46AM -0700, Nathan Chancellor wrote:
>> >> 0000017c clear_user_page:
>> >>      17c: 94 21 ff f0                  	stwu 1, -16(1)
>> >>      180: 38 80 00 80                  	li 4, 128
>> >>      184: 38 63 ff e0                  	addi 3, 3, -32
>> >>      188: 7c 89 03 a6                  	mtctr 4
>> >>      18c: 38 81 00 0f                  	addi 4, 1, 15
>> >>      190: 8c c3 00 20                  	lbzu 6, 32(3)
>> >>      194: 98 c1 00 0f                  	stb 6, 15(1)
>> >>      198: 7c 00 27 ec                  	dcbz 0, 4
>> >>      19c: 42 00 ff f4                  	bdnz .+65524
>> >
>> > Uh, yeah, well, I have no idea what clang tried here, but that won't
>> > work.  It's copying a byte from each target cache line to the stack,
>> > and then does clears the cache line containing that byte on the stack.
>> 
>> So it seems like this is a clang bug.
>> 
>> None of the distros we support use clang, but we would still like to
>> keep it working if we can.
>
> Which version?  Which versions *are* broken?

AFAIK clang 8 is the first version that we could build with, without
hacks.

>> Looking at the original patch, the only upside is that the compiler
>> can use both RA and RB to compute the address, rather than us forcing RA
>> to 0.
>> 
>> But at least with my compiler here (GCC 8 vintage) I don't actually see
>> GCC ever using both GPRs even with the patch. Or at least, there's no
>> difference before/after the patch as far as I can see.
>
> The benefit is small, certainly.

Zero is small, but I guess some things are smaller? :P

>> So my inclination is to revert the original patch. We can try again in a
>> few years :D
>> 
>> Thoughts?
>
> I think you should give the clang people time to figure out what is
> going on.

Yeah fair enough, will wait and see what their diagnosis is.

cheers