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
[off-list ref] wrote:

----- On Jan 27, 2016, at 12:24 PM, Thomas Gleixner tglx-hfZtesqFncYOwBW4kG4KsQ@public.gmane.org wrote:

quoted

On Wed, 27 Jan 2016, Josh Triplett wrote:

quoted

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.