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Date: Thu, 7 Mar 2024 19:22:36 +0200
From: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
To: Yosry Ahmed <yosryahmed@google.com>
Cc: Andrew Morton <akpm@linux-foundation.org>, 
	Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>, 
	Dave Hansen <dave.hansen@intel.com>, Peter Zijlstra <peterz@infradead.org>, 
	Andy Lutomirski <luto@kernel.org>, x86@kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [RFC PATCH 1/3] x86/mm: fix LAM cr3 mask inconsistency during
 context switch
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References: <20240307133916.3782068-1-yosryahmed@google.com>
 <20240307133916.3782068-2-yosryahmed@google.com>
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On Thu, Mar 07, 2024 at 01:39:14PM +0000, Yosry Ahmed wrote:
> In switch_mm_irqs_off(), we read the 'mm->context.lam_cr3_mask' into
> 'new_lam', which is later passed to load_new_mm_cr3(). However, there is
> a call to set_tlbstate_lam_mode() in between which will read
> 'mm->context.lam_cr3_mask' again and set 'cpu_tlbstate.lam' accordingly.
> If we race with another thread updating 'mm->context.lam_cr3_mask', the
> value in 'cpu_tlbstate.lam' could end up being different from CR3.

What other thread? LAM can only be enabled when the process has single
thread. And cannot be disabled. See MM_CONTEXT_LOCK_LAM.

> While we are at it, remove the misguiding comment that states that
> 'new_lam' may not match tlbstate_lam_cr3_mask() if a race occurs.

The comment is indeed misguiding, but for different reason. It is leftover
from the earlier version of LAM patchset.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov