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Subject: Re: [PATCH v2 1/8] powerpc/32: Add VDSO version of getcpu
To:     Michael Ellerman <mpe@ellerman.id.au>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>
Cc:     linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org
References: <cover.1566491310.git.christophe.leroy@c-s.fr>
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From:   Christophe Leroy <christophe.leroy@c-s.fr>
Message-ID: <5088c94c-3c80-0799-08a2-f1d53b95380b@c-s.fr>
Date:   Tue, 29 Oct 2019 17:10:28 +0100
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Le 18/09/2019 à 07:51, Michael Ellerman a écrit :
> 
> We are still in the middle of the years long process of removing the
> "magic" syscall on 64-bit:
> 
>    https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/powerpc/kernel/exceptions-64s.S?commit=4d856f72c10ecb060868ed10ff1b1453943fc6c8#n1578
>   
> 
> Can we not add another one on 32-bit?
> 
> Is it really such a fast path that it's worth putting a wart in the
> syscall entry like that?
> 
> Is there some other method? On s390 they have a per-cpu VDSO page, that
> would be a nice option. How we do that would be specific to a particular
> MMU, and maybe not even possible with some MMUs. So maybe that's not
> feasible.

Ok, for now I remove the fast syscall and only keep the VDSO getcpu() 
for non SMP.

I think we may in the future implement a per-cpu VDSO page that will be 
mapped to the process based on the CPU it is running on. I need to look 
at that in more details.

Christophe