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From: Quentin Perret <qperret@google.com>
To: Oliver Upton <oupton@google.com>
Cc: kvm@vger.kernel.org, Marc Zyngier <maz@kernel.org>,
	Peter Shier <pshier@google.com>, Ben Gardon <bgardon@google.com>,
	David Matlack <dmatlack@google.com>,
	Paolo Bonzini <pbonzini@redhat.com>,
	kvmarm@lists.cs.columbia.edu,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [RFC PATCH 09/17] KVM: arm64: Tear down unlinked page tables in parallel walk
Date: Thu, 21 Apr 2022 13:21:54 +0000	[thread overview]
Message-ID: <YmFactP0GnSp3vEv@google.com> (raw)
In-Reply-To: <20220415215901.1737897-10-oupton@google.com>

Hi Oliver,

On Friday 15 Apr 2022 at 21:58:53 (+0000), Oliver Upton wrote:
> Breaking a table pte is insufficient to guarantee ownership of an
> unlinked subtree. Parallel software walkers could be traversing
> substructures and changing their mappings.
> 
> Recurse through the unlinked subtree and lock all descendent ptes
> to take ownership of the subtree. Since the ptes are actually being
> evicted, return table ptes back to the table walker to ensure child
> tables are also traversed. Note that this is done both in both the
> pre-order and leaf visitors as the underlying pte remains volatile until
> it is unlinked.

Still trying to get the full picture of the series so bear with me. IIUC
the case you're dealing with here is when we're coallescing a table into
a block with concurrent walkers making changes in the sub-tree. I
believe this should happen when turning dirty logging off?

Why do we need to recursively lock the entire sub-tree at all in this
case? As long as the table is turned into a locked invalid PTE, what
concurrent walkers are doing in the sub-tree should be irrelevant no?
None of the changes they do will be made visible to the hardware anyway.
So as long as the sub-tree isn't freed under their feet (which should be
the point of the RCU protection) this should be all fine? Is there a
case where this is not actually true?

Thanks,
Quentin
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  reply	other threads:[~2022-04-21 13:22 UTC|newest]

Thread overview: 55+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-04-15 21:58 [RFC PATCH 00/17] KVM: arm64: Parallelize stage 2 fault handling Oliver Upton
2022-04-15 21:58 ` [RFC PATCH 01/17] KVM: arm64: Directly read owner id field in stage2_pte_is_counted() Oliver Upton
2022-04-15 21:58 ` [RFC PATCH 02/17] KVM: arm64: Only read the pte once per visit Oliver Upton
2022-04-21 16:12   ` Ben Gardon
2022-04-15 21:58 ` [RFC PATCH 03/17] KVM: arm64: Return the next table from map callbacks Oliver Upton
2022-04-15 21:58 ` [RFC PATCH 04/17] KVM: arm64: Protect page table traversal with RCU Oliver Upton
2022-04-19  2:55   ` Ricardo Koller
2022-04-19  3:01     ` Oliver Upton
2022-04-15 21:58 ` [RFC PATCH 05/17] KVM: arm64: Take an argument to indicate parallel walk Oliver Upton
2022-04-16 11:30   ` Marc Zyngier
2022-04-16 16:03     ` Oliver Upton
2022-04-15 21:58 ` [RFC PATCH 06/17] KVM: arm64: Implement break-before-make sequence for parallel walks Oliver Upton
2022-04-20 16:55   ` Quentin Perret
2022-04-20 17:06     ` Oliver Upton
2022-04-21 16:57   ` Ben Gardon
2022-04-21 18:52     ` Oliver Upton
2022-04-26 21:32       ` Ben Gardon
2022-04-25 15:13   ` Sean Christopherson
2022-04-25 16:53     ` Oliver Upton
2022-04-25 18:16       ` Sean Christopherson
2022-04-15 21:58 ` [RFC PATCH 07/17] KVM: arm64: Enlighten perm relax path about " Oliver Upton
2022-04-15 21:58 ` [RFC PATCH 08/17] KVM: arm64: Spin off helper for initializing table pte Oliver Upton
2022-04-15 21:58 ` [RFC PATCH 09/17] KVM: arm64: Tear down unlinked page tables in parallel walk Oliver Upton
2022-04-21 13:21   ` Quentin Perret [this message]
2022-04-21 16:40     ` Oliver Upton
2022-04-22 16:00       ` Quentin Perret
2022-04-22 20:41         ` Oliver Upton
2022-05-03 14:17           ` Quentin Perret
2022-05-04  6:03             ` Oliver Upton
2022-04-15 21:58 ` [RFC PATCH 10/17] KVM: arm64: Assume a table pte is already owned in post-order traversal Oliver Upton
2022-04-21 16:11   ` Ben Gardon
2022-04-21 17:16     ` Oliver Upton
2022-04-15 21:58 ` [RFC PATCH 11/17] KVM: arm64: Move MMU cache init/destroy into helpers Oliver Upton
2022-04-15 21:58 ` [RFC PATCH 12/17] KVM: arm64: Stuff mmu page cache in sub struct Oliver Upton
2022-04-15 21:58 ` [RFC PATCH 13/17] KVM: arm64: Setup cache for stage2 page headers Oliver Upton
2022-04-15 21:58 ` [RFC PATCH 14/17] KVM: arm64: Punt last page reference to rcu callback for parallel walk Oliver Upton
2022-04-19  2:59   ` Ricardo Koller
2022-04-19  3:09     ` Ricardo Koller
2022-04-20  0:53       ` Oliver Upton
2022-09-08  0:52         ` David Matlack
2022-04-21 16:28   ` Ben Gardon
2022-04-15 21:58 ` [RFC PATCH 15/17] KVM: arm64: Allow parallel calls to kvm_pgtable_stage2_map() Oliver Upton
2022-04-15 21:59 ` [RFC PATCH 16/17] KVM: arm64: Enable parallel stage 2 MMU faults Oliver Upton
2022-04-21 16:35   ` Ben Gardon
2022-04-21 16:46     ` Oliver Upton
2022-04-21 17:03       ` Ben Gardon
2022-04-15 21:59 ` [RFC PATCH 17/17] TESTONLY: KVM: arm64: Add super lazy accounting of stage 2 table pages Oliver Upton
2022-04-15 23:35 ` [RFC PATCH 00/17] KVM: arm64: Parallelize stage 2 fault handling David Matlack
2022-04-16  0:04   ` Oliver Upton
2022-04-21 16:43     ` David Matlack
2022-04-16  6:23 ` Oliver Upton
2022-04-19 17:57 ` Ben Gardon
2022-04-19 18:36   ` Oliver Upton
2022-04-21 16:30     ` Ben Gardon
2022-04-21 16:37       ` Paolo Bonzini

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