Re: [RFC PATCH 5/6] KVM: arm64: Optimize the stage2 map path with TLBI range instructions

[Raghavendra Rao Ananta]

Hi Oliver,

On Tue, Jan 24, 2023 at 3:25 PM Oliver Upton <oliver.upton@xxxxxxxxx> wrote:
>
> Hi Raghavendra,
>
> My comment from the previous change also applies here:
>
>   KVM: arm64: Use range-based TLBIs when collapsing hugepages
>
> On Mon, Jan 09, 2023 at 09:53:46PM +0000, Raghavendra Rao Ananta wrote:
> > Currently, when the map path of stage2 page-table coalesces a
> > bunch of pages into a hugepage, KVM invalidates the entire
> > VM's TLB entries. This would cause a perforamance penality for
> > the guest whose pages have already been coalesced earlier as they
> > would have to refill their TLB entries unnecessarily again.
>
> It is also problematic that we do this on every single fault where we
> collapse a hugepage.
>
Yes! I'll also include this description in v2.

> > Hence, if the system supports it, use __kvm_tlb_flush_range_vmid_ipa()
> > to flush only the range of pages that have been combined into
> > a hugepage, while leaving other TLB entries alone.
> >
> > Signed-off-by: Raghavendra Rao Ananta <rananta@xxxxxxxxxx>
> > ---
> >  arch/arm64/kvm/hyp/pgtable.c | 29 +++++++++++++++++++++++++----
> >  1 file changed, 25 insertions(+), 4 deletions(-)
> >
> > diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
> > index b11cf2c618a6c..099032bb01bce 100644
> > --- a/arch/arm64/kvm/hyp/pgtable.c
> > +++ b/arch/arm64/kvm/hyp/pgtable.c
> > @@ -686,6 +686,22 @@ static bool stage2_try_set_pte(const struct kvm_pgtable_visit_ctx *ctx, kvm_pte_
> >       return cmpxchg(ctx->ptep, ctx->old, new) == ctx->old;
> >  }
> >
> > +static void kvm_table_pte_flush(struct kvm_s2_mmu *mmu, u64 addr, u32 level, u32 tlb_level)
>
> Could you call this something like kvm_pgtable_flush_range() and take an
> address range as an argument? TLBIRANGE can be used outside the context
> of a table (i.e. a subset of PTEs).
>
Good idea. In that case, the function becomes very close to arm64's
implementation of kvm_flush_remote_tlbs_range() on top of David's
series [1].
Too bad, we may not be able to invoke that. I'll rename the function.

> > +{
> > +     if (system_supports_tlb_range()) {
> > +             u64 end = addr + kvm_granule_size(level);
> > +
> > +             kvm_call_hyp(__kvm_tlb_flush_range_vmid_ipa, mmu, addr, end, tlb_level);
> > +     } else {
> > +             /*
> > +              * Invalidate the whole stage-2, as we may have numerous leaf
> > +              * entries below us which would otherwise need invalidating
> > +              * individually.
> > +              */
> > +             kvm_call_hyp(__kvm_tlb_flush_vmid, mmu);
> > +     }
> > +}
> > +
> >  /**
> >   * stage2_try_break_pte() - Invalidates a pte according to the
> >   *                       'break-before-make' requirements of the
> > @@ -693,6 +709,7 @@ static bool stage2_try_set_pte(const struct kvm_pgtable_visit_ctx *ctx, kvm_pte_
> >   *
> >   * @ctx: context of the visited pte.
> >   * @mmu: stage-2 mmu
> > + * @tlb_level: The level at which the leaf pages are expected (for FEAT_TTL hint)
>
> Do we need the caller to provide the TTL hint? We already have
> ctx->level, and stage2_try_break_pte() also knows what the removed PTE
> contained (i.e. a table or a block/page).
>
ctx->level may not always translate to TTL level hint. For example,
the patch 6/6 of this series also calls stage2_try_break_pte(), but
from the very top level (level-1). In that case, the level can be
extracted from ctx, but we won't have any idea what the TTL hint is
since we won't be traversing all of the page-table. As a result, we
pass 0. However, if we are remapping a table to a bunch of pages, TTL
level hint could be just one level down.

> >   * Returns: true if the pte was successfully broken.
> >   *
> > @@ -701,7 +718,7 @@ static bool stage2_try_set_pte(const struct kvm_pgtable_visit_ctx *ctx, kvm_pte_
> >   * on the containing table page.
> >   */
> >  static bool stage2_try_break_pte(const struct kvm_pgtable_visit_ctx *ctx,
> > -                              struct kvm_s2_mmu *mmu)
> > +                              struct kvm_s2_mmu *mmu, u32 tlb_level)
> >  {
> >       struct kvm_pgtable_mm_ops *mm_ops = ctx->mm_ops;
> >
> > @@ -722,7 +739,7 @@ static bool stage2_try_break_pte(const struct kvm_pgtable_visit_ctx *ctx,
> >        * value (if any).
> >        */
> >       if (kvm_pte_table(ctx->old, ctx->level))
> > -             kvm_call_hyp(__kvm_tlb_flush_vmid, mmu);
> > +             kvm_table_pte_flush(mmu, ctx->addr, ctx->level, tlb_level);
>
> I don't think we should provide a TTL hint for a removed table. It is
> entirely possible for the unlinked table to contain a mix of blocks and
> pages, meaning there isn't a uniform table level for the whole range.
>
True, but it's particularly useful for the case where we know that a
table is going to be replaced by a block. In that case, can't we be
sure of the TTL level? Perhaps for other cases, we can supply 0. WDYT?

> >       else if (kvm_pte_valid(ctx->old))
> >               kvm_call_hyp(__kvm_tlb_flush_vmid_ipa, mmu, ctx->addr, ctx->level);
> >
> > @@ -804,7 +821,7 @@ static int stage2_map_walker_try_leaf(const struct kvm_pgtable_visit_ctx *ctx,
> >       if (!stage2_pte_needs_update(ctx->old, new))
> >               return -EAGAIN;
> >
> > -     if (!stage2_try_break_pte(ctx, data->mmu))
> > +     if (!stage2_try_break_pte(ctx, data->mmu, ctx->level))
> >               return -EAGAIN;
> >
> >       /* Perform CMOs before installation of the guest stage-2 PTE */
> > @@ -861,7 +878,11 @@ static int stage2_map_walk_leaf(const struct kvm_pgtable_visit_ctx *ctx,
> >       if (!childp)
> >               return -ENOMEM;
> >
> > -     if (!stage2_try_break_pte(ctx, data->mmu)) {
> > +     /*
> > +      * As the table will be replaced with a block, one level down would
> > +      * be the current page entries held by the table.
> > +      */
>
> This isn't necessarily true. Ignoring mixed block/pages for a moment,
> Collapsing a PUD entry into a block after dirty logging (where we mapped
> at PTE level) would imply a TTL of ctx->level + 2.
>
> But again, I think it is best to provide no hint in this case.
>
Ah, right. We could also collapse into a 1G block. No hint would be
better in that case.

Thanks,
Raghavendra

[1]: https://lore.kernel.org/linux-arm-kernel/20230119173559.2517103-1-dmatlack@xxxxxxxxxx/T/#meca775fa8accc996900d5e9caeb004f7b9774627

> --
> Thanks,
> Oliver