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: <tvbgstojgmb5jerhrdwbhknuhhbwmq2pqkb4lxyo3htdynioee@poos2yt52xg2>
Date: Thu, 7 Mar 2024 19:29:40 +0200
From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To: Yosry Ahmed <yosryahmed@...gle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, 
	Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...el.com>, 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 01:39:15PM +0000, Yosry Ahmed wrote:
> During context switching, if we are not switching to new mm and no TLB
> flush is needed, we do not write CR3. However, it is possible that a
> user thread enables LAM while a kthread is running on a different CPU
> with the old LAM CR3 mask. If the kthread context switches into any
> thread of that user process, it may not write CR3 with the new LAM mask,
> which would cause the user thread to run with a misconfigured CR3 that
> disables LAM on the CPU.

I don't think it is possible. As I said we can only enable LAM when the
process has single thread. If it enables LAM concurrently with kernel
thread and kernel thread gets control on the same CPU after the userspace
thread of the same process LAM is already going to be enabled. No need in
special handling.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov