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

From: Andrew Hunter
Date: Wed Jan 27 2016 - 13:02:14 EST

On Wed, Jan 27, 2016 at 9:36 AM, Mathieu Desnoyers
<mathieu.desnoyers@xxxxxxxxxxxx> wrote:
> ----- On Jan 27, 2016, at 12:24 PM, Thomas Gleixner tglx@xxxxxxxxxxxxx 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:
> 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.