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Message-ID: <CADroS=5JKe7XS38kO4xp51pMNpY3cCkyROLiVfPW+VEJTRydzQ@mail.gmail.com>
Date: Wed, 27 Jan 2016 10:02:03 -0800
From: Andrew Hunter <ahh@...gle.com>
To: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc: Thomas Gleixner <tglx@...utronix.de>,
Josh Triplett <josh@...htriplett.org>,
Paul Turner <pjt@...gle.com>,
Peter Zijlstra <peterz@...radead.org>,
lkml <linux-kernel@...r.kernel.org>,
linux-api <linux-api@...r.kernel.org>,
Andy Lutomirski <luto@...capital.net>,
Andi Kleen <andi@...stfloor.org>,
Dave Watson <davejwatson@...com>, Chris Lameter <cl@...ux.com>,
Ingo Molnar <mingo@...hat.com>, Ben Maurer <bmaurer@...com>,
rostedt <rostedt@...dmis.org>,
"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
Linus Torvalds <torvalds@...ux-foundation.org>,
Andrew Morton <akpm@...ux-foundation.org>,
Russell King <linux@....linux.org.uk>,
Catalin Marinas <catalin.marinas@....com>,
Will Deacon <will.deacon@....com>,
Michael Kerrisk <mtk.manpages@...il.com>
Subject: Re: [RFC PATCH v2 1/3] getcpu_cache system call: cache CPU number of
running thread
On Wed, Jan 27, 2016 at 9:36 AM, Mathieu Desnoyers
<mathieu.desnoyers@...icios.com> wrote:
> ----- On Jan 27, 2016, at 12:24 PM, Thomas Gleixner tglx@...utronix.de wrote:
>
>> On Wed, 27 Jan 2016, Josh Triplett wrote:
>>> With the dynamic allocation removed, this seems sensible to me. One
>>> minor nit: s/int32_t/uint32_t/g, since a location intended to hold a CPU
>>> number should never need to hold a negative number.
>>
>> You try to block the future of computing: https://lwn.net/Articles/638673/
>
> Besides impossible architectures, there is actually a use-case for
> signedness here. It makes it possible to initialize the cpu number
> cache to a negative value, e.g. -1, in userspace. Then, a check for
> value < 0 can be used to figure out cases where the getcpu_cache
> system call is not implemented, and where a fallback (vdso or getcpu
> syscall) needs to be used.
>
> This is why I have chosen a signed type for the cpu cache so far.
>
In our internal version of this patch (part of the RSEQ system
discussed elsewhere) we have a signed CPU id for this reason. I think
it's a good idea to keep that in userspace and it makes more sense to
match the user and kernel versions of the types.
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