linux-kernel - Re: [PATCH v4 10/12] x86,tlb: do targeted broadcast flushing from tlbbatch code

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Message-ID: <831b89b3ee65331563798b83c54a1168dfe17e16.camel@surriel.com>
Date: Mon, 13 Jan 2025 16:16:59 -0500
From: Rik van Riel <riel@...riel.com>
To: Jann Horn <jannh@...gle.com>
Cc: x86@...nel.org, linux-kernel@...r.kernel.org, bp@...en8.de, 
	peterz@...radead.org, dave.hansen@...ux.intel.com,
 zhengqi.arch@...edance.com, 	nadav.amit@...il.com, thomas.lendacky@....com,
 kernel-team@...a.com, 	linux-mm@...ck.org, akpm@...ux-foundation.org
Subject: Re: [PATCH v4 10/12] x86,tlb: do targeted broadcast flushing from
 tlbbatch code

On Mon, 2025-01-13 at 18:05 +0100, Jann Horn wrote:
> On Sun, Jan 12, 2025 at 4:55 PM Rik van Riel <riel@...riel.com>
> wrote:
> 
> > 
> > +void arch_tlbbatch_add_pending(struct arch_tlbflush_unmap_batch
> > *batch,
> > +                                            struct mm_struct *mm,
> > +                                            unsigned long uaddr)
> > +{
> > +       if (static_cpu_has(X86_FEATURE_INVLPGB) &&
> > mm_global_asid(mm)) {
> > +               u16 asid = mm_global_asid(mm);
> > +               /*
> > +                * Queue up an asynchronous invalidation. The
> > corresponding
> > +                * TLBSYNC is done in arch_tlbbatch_flush(), and
> > must be done
> > +                * on the same CPU.
> > +                */
> > +               if (!batch->used_invlpgb) {
> > +                       batch->used_invlpgb = true;
> > +                       migrate_disable();
> > +               }
> > +               invlpgb_flush_user_nr_nosync(kern_pcid(asid),
> > uaddr, 1, false);
> > +               /* Do any CPUs supporting INVLPGB need PTI? */
> > +               if (static_cpu_has(X86_FEATURE_PTI))
> > +                      
> > invlpgb_flush_user_nr_nosync(user_pcid(asid), uaddr, 1, false);
> > +       } else {
> > +               inc_mm_tlb_gen(mm);
> > +               cpumask_or(&batch->cpumask, &batch->cpumask,
> > mm_cpumask(mm));
> > +       }
> > +       mmu_notifier_arch_invalidate_secondary_tlbs(mm, 0, -1UL);
> > +}
> 
> How does this work if the MM is currently transitioning to a global
> ASID? Should the "mm_global_asid(mm)" check maybe be replaced with
> something that checks if the MM has fully transitioned to a global
> ASID, so that we keep using the classic path if there might be
> holdout
> CPUs?
> 
You are right!

If the mm is still transitioning, we should send a
TLB flush IPI, in addition to doing the broadcast shootdown.

Worst case the CPU is already using a global ASID, and
the TLB flush IPI ends up being a noop.


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