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SubjectRe: [PATCH 18/30] x86, kaiser: map virtually-addressed performance monitoring buffers
On Tue, 14 Nov 2017, Dave Hansen wrote:
> On 11/14/2017 10:20 AM, Peter Zijlstra wrote:
> > On Fri, Nov 10, 2017 at 11:31:39AM -0800, Dave Hansen wrote:
> >> static int alloc_ds_buffer(int cpu)
> >> {
> >> + struct debug_store *ds = per_cpu_ptr(&cpu_debug_store, cpu);
> >>
> >> + memset(ds, 0, sizeof(*ds));
> > Still wondering about that memset...

Sorry, my attention is far away at the moment.

>
> My guess is that it was done to mirror the zeroing done by the original
> kzalloc().

You guess right.

> But, I think you're right that it's zero'd already by virtue
> of being static:
>
> static
> DEFINE_PER_CPU_SHARED_ALIGNED_USER_MAPPED(struct debug_store,
> cpu_debug_store);
>
> I'll queue a cleanup, or update it if I re-post the set.

I was about to agree, but now I'm not so sure. I don't know much
about these PMC things, but at a glance it looks like what is reserved
by x86_reserve_hardware() may later be released by x86_release_hardware(),
and then later reserved again by x86_reserve_hardware(). And although
the static per-cpu area would be zeroed the first time, the second time
it will contain data left over from before, so really needs the memset?

Hugh

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