linux-kernel - Re: [PATCH 18/30] x86, kaiser: map virtually-addressed performance monitoring buffers

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Date:   Tue, 14 Nov 2017 11:10:23 -0800 (PST)
From:   Hugh Dickins <hughd@...gle.com>
To:     Dave Hansen <dave.hansen@...ux.intel.com>
cc:     Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org, hughd@...gle.com,
        moritz.lipp@...k.tugraz.at, daniel.gruss@...k.tugraz.at,
        michael.schwarz@...k.tugraz.at, richard.fellner@...dent.tugraz.at,
        luto@...nel.org, torvalds@...ux-foundation.org,
        keescook@...gle.com, x86@...nel.org
Subject: Re: [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