linux-kernel - Re: [RFC 2/5] x86: Store a per-cpu shadow copy of CR4

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Message-ID: <20141016082624.GH7369@worktop.fdxtended.com>
Date:	Thu, 16 Oct 2014 10:26:24 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Valdis Kletnieks <Valdis.Kletnieks@...edu>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Paul Mackerras <paulus@...ba.org>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Kees Cook <keescook@...omium.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Erik Bosman <ebn310@....vu.nl>
Subject: Re: [RFC 2/5] x86: Store a per-cpu shadow copy of CR4

On Tue, Oct 14, 2014 at 03:57:36PM -0700, Andy Lutomirski wrote:
> Context switches and TLB flushes can change individual bits of CR4.
> CR4 reads take several cycles, so store a shadow copy of CR4 in a
> per-cpu variable.
> 
> To avoid wasting a cache line, I added the CR4 shadow to
> cpu_tlbstate, which is already touched during context switches.

I'm a little confused. We should be more specific I suppose, context
switches don't always change mm, but CR4 state is per task.

>From a quick look, only switch_mm() pokes at tlb, switch_to() does not.


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