linux-kernel - Re: [RFC PATCH 2/3] x86/mm: make sure LAM is up-to-date during context switching

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Message-ID: <Zeo_9IJgcVKYUzrm@google.com>
Date: Thu, 7 Mar 2024 22:30:12 +0000
From: Yosry Ahmed <yosryahmed@...gle.com>
To: Dave Hansen <dave.hansen@...el.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>, 
	Andrew Morton <akpm@...ux-foundation.org>, Thomas Gleixner <tglx@...utronix.de>, 
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, Peter Zijlstra <peterz@...radead.org>, 
	Andy Lutomirski <luto@...nel.org>, x86@...nel.org, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 2/3] x86/mm: make sure LAM is up-to-date during
 context switching

On Thu, Mar 07, 2024 at 01:48:28PM -0800, Dave Hansen wrote:
> On 3/7/24 13:08, Yosry Ahmed wrote:
> > CPU 1                                   CPU 2
> > /* user thread running */
> > context_switch() /* to kthread */
> >                                         /* user thread enables LAM */
> >                                         context_switch()
> > context_switch() /* to user thread */
> > 
> > In this case, there are no races, but the second context switch on CPU 1
> > may not write CR3 (if TLB is up-to-date), in which case we will run the
> > user thread with CR3 having the wrong LAM mask. This could cause bigger
> > problems, right?
> 
> Yes, but this is precisely the kind of thing that we should solve with
> mm generations.  Its sole purpose is to thwart optimization where the
> mm_prev==mm_next and might not rewrite CR3.

I think it's clearer to have an explicit check for oudated LAM in
switch_mm_irqs_off(), as I mentioned in my other reply.