linux-kernel - Re: [RFC PATCH v2 1/3] getcpu_cache system call: cache CPU number of running thread

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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.