linux-kernel - Re: [PATCH v3 6/7] KVM: arm64: Add 'skip_flush' arg to stage2_put

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Message-ID: <CAJHc60wJob+VpRN-Z3VDTH1sVHSYUxPSCpyKCrC4rFBRuCcsQA@mail.gmail.com>
Date:   Tue, 16 May 2023 10:32:20 -0700
From:   Raghavendra Rao Ananta <rananta@...gle.com>
To:     Oliver Upton <oliver.upton@...ux.dev>
Cc:     Marc Zyngier <maz@...nel.org>, James Morse <james.morse@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Ricardo Koller <ricarkol@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Jing Zhang <jingzhangos@...gle.com>,
        Colton Lewis <coltonlewis@...gle.com>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH v3 6/7] KVM: arm64: Add 'skip_flush' arg to stage2_put_pte()

Hi Oliver,

On Fri, May 12, 2023 at 10:21 AM Oliver Upton <oliver.upton@...ux.dev> wrote:
>
> Hi Raghavendra,
>
> On Fri, Apr 14, 2023 at 05:29:21PM +0000, Raghavendra Rao Ananta wrote:
> > Add a 'skip_flush' argument in stage2_put_pte() to
> > control the TLB invalidations. This will be leveraged
> > by the upcoming patch to defer the individual PTE
> > invalidations until the entire walk is finished.
> >
> > No functional change intended.
> >
> > Signed-off-by: Raghavendra Rao Ananta <rananta@...gle.com>
> > ---
> >  arch/arm64/kvm/hyp/pgtable.c | 9 ++++++---
> >  1 file changed, 6 insertions(+), 3 deletions(-)
> >
> > diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
> > index b8f0dbd12f773..3f136e35feb5e 100644
> > --- a/arch/arm64/kvm/hyp/pgtable.c
> > +++ b/arch/arm64/kvm/hyp/pgtable.c
> > @@ -772,7 +772,7 @@ static void stage2_make_pte(const struct kvm_pgtable_visit_ctx *ctx, kvm_pte_t n
> >  }
> >
> >  static void stage2_put_pte(const struct kvm_pgtable_visit_ctx *ctx, struct kvm_s2_mmu *mmu,
> > -                        struct kvm_pgtable_mm_ops *mm_ops)
> > +                        struct kvm_pgtable_mm_ops *mm_ops, bool skip_flush)
>
> Assuming you are going to pull the cpufeature checks into this helper,
> it might me helpful to narrow the scope of it. 'stage2_put_pte()' sounds
> very generic, but it is about to have a very precise meaning in relation
> to kvm_pgtable_stage2_unmap().
>
> So maybe stage2_unmap_put_pte()? While at it, you'd want to have a
> shared helper for the deferral check:
>
Yeah, stage2_unmap_put_pte() sounds better. I'll change that.

> static bool stage2_unmap_defer_tlb_flush(struct kvm_pgtable *pgt)
> {
>         /* your blurb for why FWB is required too */
>         return system_supports_tlb_range() && stage2_has_fwb(pgt);
> }
>
Good idea; I can introduce the helper, now that we'll get rid of
stage2_unmap_data.skip_pte_tlbis (patch 7/7) as per your comments.
Also, since we are now making stage2_put_pte() specific to unmap, I
can also get rid of the 'skip_flush' arg and call
stage2_unmap_defer_tlb_flush() directly, or do you have a preference
for the additional arg?

Thank you.
Raghavendra
> The 'flush' part is annoying, because the exact term is an invalidation,
> but we already have that pattern in all of our TLB invalidation helpers.
>
> >  {
> >       /*
> >        * Clear the existing PTE, and perform break-before-make with
> > @@ -780,7 +780,10 @@ static void stage2_put_pte(const struct kvm_pgtable_visit_ctx *ctx, struct kvm_s
> >        */
> >       if (kvm_pte_valid(ctx->old)) {
> >               kvm_clear_pte(ctx->ptep);
> > -             kvm_call_hyp(__kvm_tlb_flush_vmid_ipa, mmu, ctx->addr, ctx->level);
> > +
> > +             if (!skip_flush)
> > +                     kvm_call_hyp(__kvm_tlb_flush_vmid_ipa, mmu,
> > +                                     ctx->addr, ctx->level);
> >       }
> >
> >       mm_ops->put_page(ctx->ptep);
> > @@ -1015,7 +1018,7 @@ static int stage2_unmap_walker(const struct kvm_pgtable_visit_ctx *ctx,
> >        * block entry and rely on the remaining portions being faulted
> >        * back lazily.
> >        */
> > -     stage2_put_pte(ctx, mmu, mm_ops);
> > +     stage2_put_pte(ctx, mmu, mm_ops, false);
> >
> >       if (need_flush && mm_ops->dcache_clean_inval_poc)
> >               mm_ops->dcache_clean_inval_poc(kvm_pte_follow(ctx->old, mm_ops),
> > --
> > 2.40.0.634.g4ca3ef3211-goog
> >
>
> --
> Thanks,
> Oliver