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Date: Fri, 8 Mar 2024 20:18:28 +0200
From: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
To: Dave Hansen <dave.hansen@intel.com>
Cc: Andy Lutomirski <luto@kernel.org>, Yosry Ahmed <yosryahmed@google.com>, 
	Andrew Morton <akpm@linux-foundation.org>, Thomas Gleixner <tglx@linutronix.de>, 
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>, 
	"Peter Zijlstra (Intel)" <peterz@infradead.org>, the arch/x86 maintainers <x86@kernel.org>, linux-mm@kvack.org, 
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [RFC PATCH 2/3] x86/mm: make sure LAM is up-to-date during
 context switching
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On Fri, Mar 08, 2024 at 07:23:58AM -0800, Dave Hansen wrote:
> On 3/7/24 17:34, Andy Lutomirski wrote:
> >> Fix this by making sure we write a new CR3 if LAM is not
> >> up-to-date. No problems were observed in practice, this was found
> >> by code inspection.
> > I think it should be fixed with a much bigger hammer: explicit IPIs.
> > Just don't ever let it get out of date, like install_ldt().
> I guess it matters whether the thing that matters is having a persistent
> inconsistency or a temporary one.  IPIs will definitely turn a permanent
> one into a temporary one.
> 
> But this is all easier to reason about if we can get rid of even the
> temporary inconsistency.
> 
> Wouldn't this be even simpler than IPIs?
> 
> static inline unsigned long set_tlbstate_lam_mode(struct mm_struct *mm)
> {
> 	unsigned long lam = READ_ONCE(mm->context.lam_cr3_mask);
> 
> +	/* LAM is for userspace only.  Ignore it for kernel threads: */
> +	if (tsk->flags & PF_KTHREAD)
> +		return 0;

I like this approach. kthread_use_mm() WARNs if it called for
non-PF_KTHREAD task, so it should be okay.

I was worried that it would also exclude io_uring, but worker threads
don't have the flag set.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov