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From: Oliver Upton <oliver.upton@linux.dev>
To: Ricardo Koller <ricarkol@google.com>
Cc: ricarkol@gmail.com, kvm@vger.kernel.org, catalin.marinas@arm.com,
	kvmarm@lists.linux.dev, andrew.jones@linux.dev,
	bgardon@google.com, maz@kernel.org, dmatlack@google.com,
	pbonzini@redhat.com, kvmarm@lists.cs.columbia.edu
Subject: Re: [RFC PATCH 02/12] KVM: arm64: Allow visiting block PTEs in post-order
Date: Mon, 14 Nov 2022 18:48:04 +0000	[thread overview]
Message-ID: <Y3KNZCLVqnFeg7hi@google.com> (raw)
In-Reply-To: <20221112081714.2169495-3-ricarkol@google.com>

On Sat, Nov 12, 2022 at 08:17:04AM +0000, Ricardo Koller wrote:
> The page table walker does not visit block PTEs in post-order. But there
> are some cases where doing so would be beneficial, for example: breaking a
> 1G block PTE into a full tree in post-order avoids visiting the new tree.
> 
> Allow post order visits of block PTEs. This will be used in a subsequent
> commit for eagerly breaking huge pages.
> 
> Signed-off-by: Ricardo Koller <ricarkol@google.com>
> ---
>  arch/arm64/include/asm/kvm_pgtable.h |  4 ++--
>  arch/arm64/kvm/hyp/nvhe/setup.c      |  2 +-
>  arch/arm64/kvm/hyp/pgtable.c         | 25 ++++++++++++-------------
>  3 files changed, 15 insertions(+), 16 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/kvm_pgtable.h b/arch/arm64/include/asm/kvm_pgtable.h
> index e2edeed462e8..d2e4a5032146 100644
> --- a/arch/arm64/include/asm/kvm_pgtable.h
> +++ b/arch/arm64/include/asm/kvm_pgtable.h
> @@ -255,7 +255,7 @@ struct kvm_pgtable {
>   *					entries.
>   * @KVM_PGTABLE_WALK_TABLE_PRE:		Visit table entries before their
>   *					children.
> - * @KVM_PGTABLE_WALK_TABLE_POST:	Visit table entries after their
> + * @KVM_PGTABLE_WALK_POST:		Visit leaf or table entries after their
>   *					children.

It is not immediately obvious from this change alone that promoting the
post-order traversal of every walker to cover leaf + table PTEs is safe.

Have you considered using a flag for just leaf post-order visits?

--
Thanks,
Oliver
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WARNING: multiple messages have this Message-ID (diff)
From: Oliver Upton <oliver.upton@linux.dev>
To: Ricardo Koller <ricarkol@google.com>
Cc: pbonzini@redhat.com, maz@kernel.org, dmatlack@google.com,
	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, kvm@vger.kernel.org,
	kvmarm@lists.linux.dev, kvmarm@lists.cs.columbia.edu,
	ricarkol@gmail.com
Subject: Re: [RFC PATCH 02/12] KVM: arm64: Allow visiting block PTEs in post-order
Date: Mon, 14 Nov 2022 18:48:04 +0000	[thread overview]
Message-ID: <Y3KNZCLVqnFeg7hi@google.com> (raw)
Message-ID: <20221114184804.OR94-bc_6kn0F2NU_GJxlG6Sg4-98rfOdEJUpgM8Tws@z> (raw)
In-Reply-To: <20221112081714.2169495-3-ricarkol@google.com>

On Sat, Nov 12, 2022 at 08:17:04AM +0000, Ricardo Koller wrote:
> The page table walker does not visit block PTEs in post-order. But there
> are some cases where doing so would be beneficial, for example: breaking a
> 1G block PTE into a full tree in post-order avoids visiting the new tree.
> 
> Allow post order visits of block PTEs. This will be used in a subsequent
> commit for eagerly breaking huge pages.
> 
> Signed-off-by: Ricardo Koller <ricarkol@google.com>
> ---
>  arch/arm64/include/asm/kvm_pgtable.h |  4 ++--
>  arch/arm64/kvm/hyp/nvhe/setup.c      |  2 +-
>  arch/arm64/kvm/hyp/pgtable.c         | 25 ++++++++++++-------------
>  3 files changed, 15 insertions(+), 16 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/kvm_pgtable.h b/arch/arm64/include/asm/kvm_pgtable.h
> index e2edeed462e8..d2e4a5032146 100644
> --- a/arch/arm64/include/asm/kvm_pgtable.h
> +++ b/arch/arm64/include/asm/kvm_pgtable.h
> @@ -255,7 +255,7 @@ struct kvm_pgtable {
>   *					entries.
>   * @KVM_PGTABLE_WALK_TABLE_PRE:		Visit table entries before their
>   *					children.
> - * @KVM_PGTABLE_WALK_TABLE_POST:	Visit table entries after their
> + * @KVM_PGTABLE_WALK_POST:		Visit leaf or table entries after their
>   *					children.

It is not immediately obvious from this change alone that promoting the
post-order traversal of every walker to cover leaf + table PTEs is safe.

Have you considered using a flag for just leaf post-order visits?

--
Thanks,
Oliver

  parent reply	other threads:[~2022-11-14 18:48 UTC|newest]

Thread overview: 46+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-11-12  8:17 [RFC PATCH 00/12] KVM: arm64: Eager huge-page splitting for dirty-logging Ricardo Koller
2022-11-12  8:17 ` Ricardo Koller
2022-11-12  8:17 ` [RFC PATCH 01/12] KVM: arm64: Relax WARN check in stage2_make_pte() Ricardo Koller
2022-11-12  8:17   ` Ricardo Koller
2022-11-14 20:59   ` Oliver Upton
2022-11-14 20:59     ` Oliver Upton
2022-11-12  8:17 ` [RFC PATCH 02/12] KVM: arm64: Allow visiting block PTEs in post-order Ricardo Koller
2022-11-12  8:17   ` Ricardo Koller
2022-11-14 18:48   ` Oliver Upton [this message]
2022-11-14 18:48     ` Oliver Upton
2023-01-13  3:44     ` Ricardo Koller
2022-11-12  8:17 ` [RFC PATCH 03/12] KVM: arm64: Add stage2_create_removed() Ricardo Koller
2022-11-12  8:17   ` Ricardo Koller
2022-11-12  8:17 ` [RFC PATCH 04/12] KVM: arm64: Add kvm_pgtable_stage2_split() Ricardo Koller
2022-11-12  8:17   ` Ricardo Koller
2022-11-14 20:54   ` Oliver Upton
2022-11-14 20:54     ` Oliver Upton
2022-11-15 23:03     ` Ricardo Koller
2022-11-15 23:03       ` Ricardo Koller
2022-11-15 23:27       ` Ricardo Koller
2022-11-15 23:27         ` Ricardo Koller
2022-11-15 23:54         ` Oliver Upton
2022-11-15 23:54           ` Oliver Upton
2022-11-17 21:50           ` Ricardo Koller
2022-11-17 21:50             ` Ricardo Koller
2022-11-12  8:17 ` [RFC PATCH 05/12] arm64: Add a capability for FEAT_BBM level 2 Ricardo Koller
2022-11-12  8:17   ` Ricardo Koller
2022-11-12  8:17 ` [RFC PATCH 06/12] KVM: arm64: Split block PTEs without using break-before-make Ricardo Koller
2022-11-12  8:17   ` Ricardo Koller
2022-11-14 18:56   ` Oliver Upton
2022-11-14 18:56     ` Oliver Upton
2022-11-12  8:17 ` [RFC PATCH 07/12] KVM: arm64: Refactor kvm_arch_commit_memory_region() Ricardo Koller
2022-11-12  8:17   ` Ricardo Koller
2022-11-12  8:17 ` [RFC PATCH 08/12] KVM: arm64: Add kvm_uninit_stage2_mmu() Ricardo Koller
2022-11-12  8:17   ` Ricardo Koller
2022-11-12  8:17 ` [RFC PATCH 09/12] KVM: arm64: Split huge pages when dirty logging is enabled Ricardo Koller
2022-11-12  8:17   ` Ricardo Koller
2022-11-12  8:17 ` [RFC PATCH 10/12] KVM: arm64: Open-code kvm_mmu_write_protect_pt_masked() Ricardo Koller
2022-11-12  8:17   ` Ricardo Koller
2022-11-12  8:17 ` [RFC PATCH 11/12] KVM: arm64: Split huge pages during KVM_CLEAR_DIRTY_LOG Ricardo Koller
2022-11-12  8:17   ` Ricardo Koller
2022-11-12  8:17 ` [RFC PATCH 12/12] KVM: arm64: Use local TLBI on permission relaxation Ricardo Koller
2022-11-12  8:17   ` Ricardo Koller
2022-11-14 18:42 ` [RFC PATCH 00/12] KVM: arm64: Eager huge-page splitting for dirty-logging Oliver Upton
2022-11-14 18:42   ` Oliver Upton
2023-01-13  3:42   ` Ricardo Koller

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