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From: Marc Zyngier <maz@kernel.org>
To: Ricardo Koller <ricarkol@google.com>
Cc: pbonzini@redhat.com, oupton@google.com, yuzenghui@huawei.com,
	dmatlack@google.com, kvm@vger.kernel.org, kvmarm@lists.linux.dev,
	qperret@google.com, catalin.marinas@arm.com,
	andrew.jones@linux.dev, seanjc@google.com,
	alexandru.elisei@arm.com, suzuki.poulose@arm.com,
	eric.auger@redhat.com, gshan@redhat.com, reijiw@google.com,
	rananta@google.com, bgardon@google.com, ricarkol@gmail.com
Subject: Re: [PATCH v6 12/12] KVM: arm64: Use local TLBI on permission relaxation
Date: Sun, 12 Mar 2023 13:22:40 +0000	[thread overview]
Message-ID: <87356a5ckf.wl-maz@kernel.org> (raw)
In-Reply-To: <20230307034555.39733-13-ricarkol@google.com>

On Tue, 07 Mar 2023 03:45:55 +0000,
Ricardo Koller <ricarkol@google.com> wrote:
> 
> From: Marc Zyngier <maz@kernel.org>

Thanks for writing a commit message for my hacks!

> 
> Broadcasted TLB invalidations (TLBI) are usually less performant than

More precisely, TLBIs targeting the Inner Shareable domain. Also,
's/broadcasted/broadcast/', as this is an adjective and not a verb
indicative of the past tense..

> their local variant. In particular, we observed some implementations

non-shareable rather than local. 'Local' has all sort of odd
implementation specific meanings (local to *what* is the usual
question that follows...).

> that take millliseconds to complete parallel broadcasted TLBIs.
> 
> It's safe to use local, non-shareable, TLBIs when relaxing permissions

s/local//

> on a PTE in the KVM case for a couple of reasons. First, according to
> the ARM Arm (DDI 0487H.a D5-4913), permission relaxation does not need
> break-before-make.

This requires some more details, and references to the latest revision
of the ARM ARM (0487I.a). In that particular revision, the relevant
information is contained in D8.13.1 "Using break-before-make when
updating translation table entries", and more importantly in the rule
R_WHZWS, which states that only a change of output address or block
size require a BBM.

> Second, the VTTBR_EL2.CnP==0 case, where each PE
> has its own TLB entry for the same page, is tolerated correctly by KVM
> when doing permission relaxation. Not having changes broadcasted to
> all PEs is correct for this case, as it's safe to have other PEs fault
> on permission on the same page.

I'm not sure mentioning CnP is relevant here. If CnP==1, the TLBI will
nuke the TLB visible by the sibling PE, but not any other. So this is
always a partial TLB invalidation, irrespective of CnP.

Thanks,

	M.

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

  reply	other threads:[~2023-03-12 13:24 UTC|newest]

Thread overview: 35+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2023-03-07  3:45 [PATCH v6 00/12] Implement Eager Page Splitting for ARM Ricardo Koller
2023-03-07  3:45 ` [PATCH v6 01/12] KVM: arm64: Rename free_removed to free_unlinked Ricardo Koller
2023-03-07  3:45 ` [PATCH v6 02/12] KVM: arm64: Add KVM_PGTABLE_WALK ctx->flags for skipping BBM and CMO Ricardo Koller
2023-03-12 10:49   ` Marc Zyngier
2023-03-13 18:49     ` Ricardo Koller
2023-03-07  3:45 ` [PATCH v6 03/12] KVM: arm64: Add helper for creating unlinked stage2 subtrees Ricardo Koller
2023-03-12 11:06   ` Marc Zyngier
2023-03-13 22:23     ` Ricardo Koller
2023-03-07  3:45 ` [PATCH v6 04/12] KVM: arm64: Add kvm_pgtable_stage2_split() Ricardo Koller
2023-03-12 11:35   ` Marc Zyngier
2023-03-13 23:58     ` Ricardo Koller
2023-03-15 18:09       ` Marc Zyngier
2023-03-15 18:51         ` Ricardo Koller
2023-03-07  3:45 ` [PATCH v6 05/12] KVM: arm64: Refactor kvm_arch_commit_memory_region() Ricardo Koller
2023-03-07  3:45 ` [PATCH v6 06/12] KVM: arm64: Add kvm_uninit_stage2_mmu() Ricardo Koller
2023-03-07  3:45 ` [PATCH v6 07/12] KVM: arm64: Export kvm_are_all_memslots_empty() Ricardo Koller
2023-03-12 11:39   ` Marc Zyngier
2023-03-13 15:18     ` Sean Christopherson
2023-03-14 10:18       ` Marc Zyngier
2023-03-15 21:00         ` Sean Christopherson
2023-03-07  3:45 ` [PATCH v6 08/12] KVM: arm64: Add KVM_CAP_ARM_EAGER_SPLIT_CHUNK_SIZE Ricardo Koller
2023-03-12 11:56   ` Marc Zyngier
2023-03-24  7:41     ` Ricardo Koller
2023-03-29  4:50   ` Reiji Watanabe
2023-04-10 20:04     ` Ricardo Koller
2023-03-07  3:45 ` [PATCH v6 09/12] KVM: arm64: Split huge pages when dirty logging is enabled Ricardo Koller
2023-03-12 12:54   ` Marc Zyngier
2023-04-10 18:32     ` Ricardo Koller
2023-03-07  3:45 ` [PATCH v6 10/12] KVM: arm64: Open-code kvm_mmu_write_protect_pt_masked() Ricardo Koller
2023-03-07  3:45 ` [PATCH v6 11/12] KVM: arm64: Split huge pages during KVM_CLEAR_DIRTY_LOG Ricardo Koller
2023-03-12 13:01   ` Marc Zyngier
2023-04-10 18:26     ` Ricardo Koller
2023-03-07  3:45 ` [PATCH v6 12/12] KVM: arm64: Use local TLBI on permission relaxation Ricardo Koller
2023-03-12 13:22   ` Marc Zyngier [this message]
2023-04-10 18:22     ` Ricardo Koller

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