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: <e5tjo62yrvmviixily6ramjvnqqmokvayyr6k7c7qzrczcx4yd@yupixsoj53vm>
Date: Mon, 11 Mar 2024 14:42:17 +0200
From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To: Yosry Ahmed <yosryahmed@...gle.com>
Cc: Dave Hansen <dave.hansen@...el.com>, Andy Lutomirski <luto@...nel.org>, 
	Andrew Morton <akpm@...ux-foundation.org>, Thomas Gleixner <tglx@...utronix.de>, 
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	"Peter Zijlstra (Intel)" <peterz@...radead.org>, the arch/x86 maintainers <x86@...nel.org>, linux-mm@...ck.org, 
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 2/3] x86/mm: make sure LAM is up-to-date during
 context switching

On Sat, Mar 09, 2024 at 01:37:06PM -0800, Yosry Ahmed wrote:
> On Sat, Mar 9, 2024 at 8:34 AM Kirill A. Shutemov
> <kirill.shutemov@...ux.intel.com> wrote:
> >
> > On Sat, Mar 09, 2024 at 02:19:19AM +0000, Yosry Ahmed wrote:
> > > I don't see how skipping set_tlbstate_lam_mode() for kthreads fixes this
> > > problem. Do you mind elaborating?
> >
> > Define what problem is.
> >
> > Yes, in this scenario kthread gets more permissive LAM mode than it needs.
> > But nothing breaks.
> 
> 
> The problem here is not how the kthread runs at all. It is the fact
> that if that kthread context switches into the user process that has
> enabled LAM, it may not update CR3 because the mm doesn't change.
> switch_mm_irqs_off() will only update CR3 in this case if there is a
> pending TLB flush. Otherwise, we just return, even if the LAM for this
> mm has changed.
> 
> This can cause the process that has enabled LAM to run with LAM
> disabled and fault on tagged addresses, right? Did I miss something?

You are right. I think IPI is the way to go.

Will you prepare a patch?

-- 
  Kiryl Shutsemau / Kirill A. Shutemov