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

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Message-ID: <20171115094132.ur4evzvsxvxdlivl@hirez.programming.kicks-ass.net>
Date:   Wed, 15 Nov 2017 10:41:32 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Hugh Dickins <hughd@...gle.com>
Cc:     Dave Hansen <dave.hansen@...ux.intel.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        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, Nov 14, 2017 at 11:10:23AM -0800, Hugh Dickins wrote:
> 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?

Ah, yes. It does get reused. I think its still fine, but yes lets keep
it. Better safe than sorry and its not a hot path in any case.

Thanks!