linux-kernel - Re: [PATCHv11 04/16] x86/mm: Handle LAM on context switch

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Message-ID: <20221107171419.k33qd4rz3tyfrovs@box.shutemov.name>
Date:   Mon, 7 Nov 2022 20:14:19 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>, x86@...nel.org,
        Kostya Serebryany <kcc@...gle.com>,
        Andrey Ryabinin <ryabinin.a.a@...il.com>,
        Andrey Konovalov <andreyknvl@...il.com>,
        Alexander Potapenko <glider@...gle.com>,
        Taras Madan <tarasmadan@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        "H . J . Lu" <hjl.tools@...il.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Rick Edgecombe <rick.p.edgecombe@...el.com>,
        Bharata B Rao <bharata@....com>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        Ashok Raj <ashok.raj@...el.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCHv11 04/16] x86/mm: Handle LAM on context switch

On Mon, Nov 07, 2022 at 06:58:59AM -0800, Andy Lutomirski wrote:
> > @@ -554,6 +561,7 @@ void switch_mm_irqs_off(struct mm_struct *prev, struct mm_struct *next,
> >   	if (real_prev == next) {
> >   		VM_WARN_ON(this_cpu_read(cpu_tlbstate.ctxs[prev_asid].ctx_id) !=
> >   			   next->context.ctx_id);
> > +		VM_WARN_ON(prev_lam != new_lam);
> 
> What prevents this warning from firing if a remote cpu does
> prctl_enable_tagged_addr() and this cpu hits this code path before getting
> the LAM-enabling IPI?  Conceptually this would be like if we asserted that
> LDTR matched the mm_context's ldt setting in this code path.
> 
> I think (haven't really verified) that you can fix this by removing the
> warning and adding a comment explaining that CR3 can be out of sync due to a
> race against changes to LAM settings.  I don't think there's any way to
> eliminate the race -- there is no lock you can take while changing lam that
> prevents a remote CPU from switching mm or scheduling.

Something like this?

diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
index d6c9c15d2ad2..c6cac1a1bc64 100644
--- a/arch/x86/mm/tlb.c
+++ b/arch/x86/mm/tlb.c
@@ -561,7 +561,15 @@ void switch_mm_irqs_off(struct mm_struct *prev, struct mm_struct *next,
 	if (real_prev == next) {
 		VM_WARN_ON(this_cpu_read(cpu_tlbstate.ctxs[prev_asid].ctx_id) !=
 			   next->context.ctx_id);
-		VM_WARN_ON(prev_lam != new_lam);
+
+		/*
+		 * 'prev_lam' does not necessary match 'new_lam' here. In case
+		 * of race with LAM enabling, the updated 'lam_cr3_mask' can be
+		 * been before LAM-enabling IPI kicks in.
+		 *
+		 * The race is harmless: it is okay to update CR3 with new LAM
+		 * mode. The IPI will rewrite CR3 shortly.
+		 */
 
 		/*
 		 * Even in lazy TLB mode, the CPU should stay set in the
-- 
  Kiryl Shutsemau / Kirill A. Shutemov