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

From: Oliver Upton <oliver.upton@linux.dev>
To: Raghavendra Rao Ananta <rananta@google.com>
Cc: Marc Zyngier <maz@kernel.org>, James Morse <james.morse@arm.com>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	Ricardo Koller <ricarkol@google.com>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Jing Zhang <jingzhangos@google.com>,
	Colton Lewis <coltonlewis@google.com>,
	linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev,
	linux-kernel@vger.kernel.org, kvm@vger.kernel.org
Subject: Re: [PATCH v3 7/7] KVM: arm64: Use TLBI range-based intructions for unmap
Date: Tue, 16 May 2023 18:46:39 +0000	[thread overview]
Message-ID: <ZGPPj1AXS0Uah2Ug@linux.dev> (raw)
In-Reply-To: <CAJHc60wUu3xB4J8oJ+FCxerDad1TzZLCMgHYGFfv0K-hzC0qmw@mail.gmail.com>

On Tue, May 16, 2023 at 10:21:33AM -0700, Raghavendra Rao Ananta wrote:
> On Fri, May 12, 2023 at 10:02 AM Oliver Upton <oliver.upton@linux.dev> wrote:
> > >  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?

Ah, right, these were recently changed to rely on alternative patching
in commit 21fb26bfb01f ("arm64: alternatives: add alternative_has_feature_*()").
Even still, the significance remains as the alternative patching
completely eliminates a conditional branch on the presence of a
particular feature.

Initializing a local with the presence/absence of a feature defeats such
an optimization.

-- 
Thanks,
Oliver