Re: [PATCH v2] vdso: Remove struct getcpu_cache

[Andy Lutomirski <luto@kernel.org>]

On Mon, Oct 13, 2025 at 7:07 AM Dave Hansen <dave.hansen@intel.com> wrote:
>
> On 10/13/25 02:20, Thomas Weißschuh wrote:
> > -int __vdso_getcpu(unsigned int *cpu, unsigned int *node, struct getcpu_cache *unused);
> > -int __vdso_getcpu(unsigned int *cpu, unsigned int *node, struct getcpu_cache *unused)
> > +int __vdso_getcpu(unsigned int *cpu, unsigned int *node, void *unused);
> > +int __vdso_getcpu(unsigned int *cpu, unsigned int *node, void *unused)
> >  {
> >       int cpu_id;
>
> It would ideally be nice to have a _bit_ more history on this about
> how it became unused any why there is such high confidence that
> userspace never tries to use it.

The theory is that people thought that getcpu was going to be kind of
slow, so userspace would allocate a little cache (IIRC per-thread) and
pass it in, and the vDSO would do, well, something clever to return
the right value.  The something clever was probably based on the idea
that you can't actually tell (in general) if the return value from
getcpu is stale, since you might well get migrated right as the
function returns anyway, so the cache could be something silly like
(jiffies, cpu).

I don't actually remember whether the kernel ever used this.  It's
possible that there are ancient kernels where passing a wild, non-null
pointer would blow up.  But it's certainly safe to pass null, and it's
certainly safe for the kernel to ignore the parameter.

--Andy

>
> Let's say someone comes along in a few years and wants to use this
> 'unused' parameter. Could they?
>