linux-kernel - Re: [PATCH] powerpc/mm: some cleanup of do_page

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Date:   Tue, 18 Apr 2017 15:48:11 +1000
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Christophe Leroy <christophe.leroy@....fr>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Scott Wood <oss@...error.net>
Cc:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH] powerpc/mm: some cleanup of do_page_fault()

Christophe Leroy <christophe.leroy@....fr> writes:

> This patch is a bunch of small cleanups of the do_page_fault()
> function:
> 1/ Function store_updates_sp() checks whether the faulting
> instruction is a store updating r1. Therefore we can limit its calls
> to stores exceptions
> 2/ Only the get_user() in store_updates_sp() has to be done outside
> the mm semaphore. All the comparison can be done within the semaphore,
> so only when really needed.
> 3/ As we got a DSI exception, the address pointed by regs->nip is
> obviously valid, otherwise we would have had a instruction exception.
> So __get_user() can be used instead of get_user()
> 4/ Replaced one duplicate 'trap == 0x400' by 'is_exec'
> 5/ Created a 'is_user = user_mode(regs)' and replaced all
> 'user_mode(regs)'.
> Analysis of the assembly code shows that when using user_mode(regs),
> at least the 'andi. r7,r8,16384' is redone several times, and also
> the 'lwz r8,132(r31)' at times. With the new form, the 'is_user'
> is mapped to cr4, then all further use of is_user results in just
> things like 'beq cr4,218 <do_page_fault+0x218>'
> 6/ The 8xx has a dedicated exception for breakpoints, that directly
> calls do_break()

Those are probably all OK, if you send them as separate patches.

> 7/ stdu and stdux only exist on PPC64, so no need to check for them
> on PPC32

I don't love that, because it adds two ifdefs in the C code and the gain
must be pretty small.

cheers