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Date:   Thu, 03 Aug 2023 00:28:12 +0100
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From:   Marc Zyngier <maz@kernel.org>
To:     Raghavendra Rao Ananta <rananta@google.com>
Cc:     Oliver Upton <oliver.upton@linux.dev>,
        James Morse <james.morse@arm.com>,
        Suzuki K Poulose <suzuki.poulose@arm.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Sean Christopherson <seanjc@google.com>,
        Huacai Chen <chenhuacai@kernel.org>,
        Zenghui Yu <yuzenghui@huawei.com>,
        Anup Patel <anup@brainfault.org>,
        Atish Patra <atishp@atishpatra.org>,
        Jing Zhang <jingzhangos@google.com>,
        Reiji Watanabe <reijiw@google.com>,
        Colton Lewis <coltonlewis@google.com>,
        David Matlack <dmatlack@google.com>,
        linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev,
        linux-mips@vger.kernel.org, kvm-riscv@lists.infradead.org,
        linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org,
        kvm@vger.kernel.org
Subject: Re: [PATCH v7 12/12] KVM: arm64: Use TLBI range-based intructions for unmap
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On Mon, 31 Jul 2023 19:26:09 +0100,
Raghavendra Rao Ananta <rananta@google.com> wrote:
>=20
> On Thu, Jul 27, 2023 at 6:12=E2=80=AFAM Marc Zyngier <maz@kernel.org> wro=
te:
> >
> > On Sat, 22 Jul 2023 03:22:51 +0100,
> > Raghavendra Rao Ananta <rananta@google.com> 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 on some systems.
> > >
> > > 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 deferred TLB invalidation on the system supporting FWB,
> > > as the optimization is entirely pointless when the unmap walker
> > > needs to perform CMOs.
> > >
> > > Rename stage2_put_pte() to stage2_unmap_put_pte() as the function
> > > now serves the stage-2 unmap walker specifically, rather than
> > > acting generic.
> > >
> > > Signed-off-by: Raghavendra Rao Ananta <rananta@google.com>
> > > ---
> > >  arch/arm64/kvm/hyp/pgtable.c | 67 +++++++++++++++++++++++++++++++---=
--
> > >  1 file changed, 58 insertions(+), 9 deletions(-)
> > >
> > > diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtabl=
e.c
> > > index 5ef098af1736..cf88933a2ea0 100644
> > > --- a/arch/arm64/kvm/hyp/pgtable.c
> > > +++ b/arch/arm64/kvm/hyp/pgtable.c
> > > @@ -831,16 +831,54 @@ static void stage2_make_pte(const struct kvm_pg=
table_visit_ctx *ctx, kvm_pte_t n
> > >       smp_store_release(ctx->ptep, new);
> > >  }
> > >
> > > -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 stage2_unmap_data {
> > > +     struct kvm_pgtable *pgt;
> > > +     bool defer_tlb_flush_init;
> > > +};
> > > +
> > > +static bool __stage2_unmap_defer_tlb_flush(struct kvm_pgtable *pgt)
> > > +{
> > > +     /*
> > > +      * If FEAT_TLBIRANGE is implemented, defer the individual
> > > +      * TLB invalidations until the entire walk is finished, and
> > > +      * then use the range-based TLBI instructions to do the
> > > +      * invalidations. Condition deferred TLB invalidation on the
> > > +      * system supporting FWB, as the optimization is entirely
> > > +      * pointless when the unmap walker needs to perform CMOs.
> > > +      */
> > > +     return system_supports_tlb_range() && stage2_has_fwb(pgt);
> > > +}
> > > +
> > > +static bool stage2_unmap_defer_tlb_flush(struct stage2_unmap_data *u=
nmap_data)
> > > +{
> > > +     bool defer_tlb_flush =3D __stage2_unmap_defer_tlb_flush(unmap_d=
ata->pgt);
> > > +
> > > +     /*
> > > +      * Since __stage2_unmap_defer_tlb_flush() is based on alternati=
ve
> > > +      * patching and the TLBIs' operations behavior depend on this,
> > > +      * track if there's any change in the state during the unmap se=
quence.
> > > +      */
> > > +     WARN_ON(unmap_data->defer_tlb_flush_init !=3D defer_tlb_flush);
> > > +     return defer_tlb_flush;
> >
> > I really don't understand what you're testing here. The ability to
> > defer TLB invalidation is a function of the system capabilities
> > (range+FWB) and a single flag that is only set on the host for pKVM.
> >
> > How could that change in the middle of the life of the system? if
> > further begs the question about the need for the unmap_data data
> > structure.
> >
> > It looks to me that we could simply pass the pgt pointer around and be
> > done with it. Am I missing something obvious?
> >
> From one of the previous comments [1] (used in a different context),
> I'm given to understand that since these feature checks are governed
> by alternative patching, they can potentially change (at runtime?). Is
> that not the case and I have misunderstood the idea in comment [1]
> entirely? Is it solely used for optimization purposes and set only
> once?

Alternative patching, just like the static branches used to implement
the capability stuff, is a one way street. At the point where KVM is
initialised, these configurations are set in stone, and there is no
going back.

> If that's the case, I can get rid of the WARN_ON() and unmap_data.

yes, please.

Thanks,

	M.

--=20
Without deviation from the norm, progress is not possible.