linux-kernel - Re: [PATCH RESEND v3 6/6] powerpc/signal: Use unsafe_copy_siginfo_to

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Date:   Mon, 13 Sep 2021 19:19:26 +0200
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>, hch@...radead.org,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH RESEND v3 6/6] powerpc/signal: Use
 unsafe_copy_siginfo_to_user()



Le 13/09/2021 à 18:21, Eric W. Biederman a écrit :
> ebiederm@...ssion.com (Eric W. Biederman) writes:
> 
>> Christophe Leroy <christophe.leroy@...roup.eu> writes:
>>
>>> Use unsafe_copy_siginfo_to_user() in order to do the copy
>>> within the user access block.
>>>
>>> On an mpc 8321 (book3s/32) the improvment is about 5% on a process
>>> sending a signal to itself.
> 
> If you can't make function calls from an unsafe macro there is another
> way to handle this that doesn't require everything to be inline.
> 
>  From a safety perspective it is probably even a better approach.

Yes but that's exactly what I wanted to avoid for the native ppc32 case: 
this double hop means useless pressure on the cache. The siginfo_t 
structure is 128 bytes large, that means 8 lines of cache on powerpc 8xx.

But maybe it is acceptable to do that only for the compat case. Let me 
think about it, it might be quite easy.

Christophe