Re: [PATCH v3 7/7] KVM: arm64: Use TLBI range-based intructions for unmap

[Raghavendra Rao Ananta]

On Fri, May 12, 2023 at 10:02 AM Oliver Upton <oliver.upton@xxxxxxxxx> wrote:
>
> Hi Raghavendra,
>
> On Fri, Apr 14, 2023 at 05:29:22PM +0000, Raghavendra Rao Ananta wrote:
> > The current implementation of the stage-2 unmap walker traverses
> > the given range and, as a part of break-before-make, performs
> > TLB invalidations with a DSB for every PTE. A multitude of this
> > combination could cause a performance bottleneck.
> >
> > Hence, if the system supports FEAT_TLBIRANGE, defer the TLB
> > invalidations until the entire walk is finished, and then
> > use range-based instructions to invalidate the TLBs in one go.
> > Condition this upon S2FWB in order to avoid walking the page-table
> > again to perform the CMOs after issuing the TLBI.
> >
> > Signed-off-by: Raghavendra Rao Ananta <rananta@xxxxxxxxxx>
> > Suggested-by: Oliver Upton <oliver.upton@xxxxxxxxx>
> > ---
> >  arch/arm64/kvm/hyp/pgtable.c | 33 +++++++++++++++++++++++++++++----
> >  1 file changed, 29 insertions(+), 4 deletions(-)
> >
> > diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
> > index 3f136e35feb5e..bcb748e3566c7 100644
> > --- a/arch/arm64/kvm/hyp/pgtable.c
> > +++ b/arch/arm64/kvm/hyp/pgtable.c
> > @@ -987,10 +987,16 @@ int kvm_pgtable_stage2_set_owner(struct kvm_pgtable *pgt, u64 addr, u64 size,
> >       return ret;
> >  }
> >
> > +struct stage2_unmap_data {
> > +     struct kvm_pgtable *pgt;
> > +     bool skip_pte_tlbis;
> > +};
> > +
> >  static int stage2_unmap_walker(const struct kvm_pgtable_visit_ctx *ctx,
> >                              enum kvm_pgtable_walk_flags visit)
> >  {
> > -     struct kvm_pgtable *pgt = ctx->arg;
> > +     struct stage2_unmap_data *unmap_data = ctx->arg;
> > +     struct kvm_pgtable *pgt = unmap_data->pgt;
> >       struct kvm_s2_mmu *mmu = pgt->mmu;
> >       struct kvm_pgtable_mm_ops *mm_ops = ctx->mm_ops;
> >       kvm_pte_t *childp = NULL;
> > @@ -1018,7 +1024,7 @@ 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, false);
> > +     stage2_put_pte(ctx, mmu, mm_ops, unmap_data->skip_pte_tlbis);
> >
> >       if (need_flush && mm_ops->dcache_clean_inval_poc)
> >               mm_ops->dcache_clean_inval_poc(kvm_pte_follow(ctx->old, mm_ops),
> > @@ -1032,13 +1038,32 @@ static int stage2_unmap_walker(const struct kvm_pgtable_visit_ctx *ctx,
> >
> >  int kvm_pgtable_stage2_unmap(struct kvm_pgtable *pgt, u64 addr, u64 size)
> >  {
> > +     int ret;
> > +     struct stage2_unmap_data unmap_data = {
> > +             .pgt = pgt,
> > +             /*
> > +              * If FEAT_TLBIRANGE is implemented, defer the individial PTE
> > +              * TLB invalidations until the entire walk is finished, and
> > +              * then use the range-based TLBI instructions to do the
> > +              * invalidations. Condition this upon S2FWB in order to avoid
> > +              * a page-table walk again to perform the CMOs after TLBI.
> > +              */
> > +             .skip_pte_tlbis = system_supports_tlb_range() &&
> > +                                     stage2_has_fwb(pgt),
>
> Why can't the underlying walker just call these two helpers directly?
> There are static keys behind these...
>
I wasn't aware of that. Although, I tried to look into the
definitions, but couldn't understand how static keys are at play here.
By any chance are you referring to the alternative_has_feature_*()
implementations when checking for cpu capabilities?

Thank you.
Raghavendra

> > +     };
> >       struct kvm_pgtable_walker walker = {
> >               .cb     = stage2_unmap_walker,
> > -             .arg    = pgt,
> > +             .arg    = &unmap_data,
> >               .flags  = KVM_PGTABLE_WALK_LEAF | KVM_PGTABLE_WALK_TABLE_POST,
> >       };
> >
> > -     return kvm_pgtable_walk(pgt, addr, size, &walker);
> > +     ret = kvm_pgtable_walk(pgt, addr, size, &walker);
> > +     if (unmap_data.skip_pte_tlbis)
> > +             /* Perform the deferred TLB invalidations */
> > +             kvm_call_hyp(__kvm_tlb_flush_vmid_range, pgt->mmu,
> > +                             addr, addr + size);
> > +
> > +     return ret;
> >  }
> >
> >  struct stage2_attr_data {
> > --
> > 2.40.0.634.g4ca3ef3211-goog
> >
>
> --
> Thanks,
> Oliver