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

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Date: Thu, 7 Mar 2024 13:48:28 -0800
From: Dave Hansen <dave.hansen@...el.com>
To: Yosry Ahmed <yosryahmed@...gle.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 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.