From: Sean Christopherson <seanjc@google.com>
To: Oliver Upton <oliver.upton@linux.dev>
Cc: Marc Zyngier <maz@kernel.org>, James Morse <james.morse@arm.com>,
Alexandru Elisei <alexandru.elisei@arm.com>,
linux-arm-kernel@lists.infradead.org,
kvmarm@lists.cs.columbia.edu, kvm@vger.kernel.org,
Reiji Watanabe <reijiw@google.com>,
Ricardo Koller <ricarkol@google.com>,
David Matlack <dmatlack@google.com>,
Quentin Perret <qperret@google.com>,
Ben Gardon <bgardon@google.com>, Gavin Shan <gshan@redhat.com>,
Peter Xu <peterx@redhat.com>, Will Deacon <will@kernel.org>,
kvmarm@lists.linux.dev
Subject: Re: [PATCH v3 09/15] KVM: arm64: Free removed stage-2 tables in RCU callback
Date: Tue, 1 Nov 2022 21:07:30 +0000 [thread overview]
Message-ID: <Y2GKkvMWTHfuPf4Y@google.com> (raw)
In-Reply-To: <Y2GFliAVxui9VyK2@google.com>
On Tue, Nov 01, 2022, Oliver Upton wrote:
> On Tue, Nov 01, 2022 at 08:28:04PM +0000, Sean Christopherson wrote:
> > On Thu, Oct 27, 2022, Oliver Upton wrote:
> > > There is no real urgency to free a stage-2 subtree that was pruned.
> > > Nonetheless, KVM does the tear down in the stage-2 fault path while
> > > holding the MMU lock.
> > >
>
> [ copy ]
>
> > This is _very_ misleading. The above paints RCU as an optimization of sorts to
> > avoid doing work while holding mmu_lock. Freeing page tables in an RCU callback
> > is _required_ for correctness when allowing parallel page faults to remove page
> > tables, as holding mmu_lock for read in that case doesn't ensure no other CPU is
> > accessing and/or holds a reference to the to-be-freed page table.
>
> Agree, but it is still important to reason about what is changing here
> too. Moving work out of the vCPU fault path _is_ valuable, though
> ancillary to the correctness requirements.
Sure, but that's at best a footnote. Similar to protecting freeing, RCU isn't
the only option for moving work out of the vCPU fault path. In fact, it's probably
one of the worst options because RCU callbacks run with soft IRQs disabled, i.e.
doing _too_ much in a RCU callback is a real problem. If RCU weren't being used
to protect readers, deferring freeing via a workqueue, kthread, etc... would work
just as well, if not better.
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next prev parent reply other threads:[~2022-11-01 21:09 UTC|newest]
Thread overview: 24+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-10-27 22:17 [PATCH v3 00/15] KVM: arm64: Parallel stage-2 fault handling Oliver Upton
2022-10-27 22:17 ` [PATCH v3 01/15] KVM: arm64: Combine visitor arguments into a context structure Oliver Upton
2022-10-27 22:17 ` [PATCH v3 02/15] KVM: arm64: Stash observed pte value in visitor context Oliver Upton
2022-10-27 22:17 ` [PATCH v3 03/15] KVM: arm64: Pass mm_ops through the " Oliver Upton
2022-10-27 22:17 ` [PATCH v3 04/15] KVM: arm64: Don't pass kvm_pgtable through kvm_pgtable_walk_data Oliver Upton
2022-10-27 22:17 ` [PATCH v3 05/15] KVM: arm64: Add a helper to tear down unlinked stage-2 subtrees Oliver Upton
2022-10-27 22:17 ` [PATCH v3 06/15] KVM: arm64: Tear down unlinked stage-2 subtree after break-before-make Oliver Upton
2022-10-27 22:17 ` [PATCH v3 07/15] KVM: arm64: Use an opaque type for pteps Oliver Upton
2022-10-27 22:17 ` [PATCH v3 08/15] KVM: arm64: Protect stage-2 traversal with RCU Oliver Upton
2022-10-28 9:19 ` Oliver Upton
2022-11-01 20:22 ` Sean Christopherson
2022-10-27 22:17 ` [PATCH v3 09/15] KVM: arm64: Free removed stage-2 tables in RCU callback Oliver Upton
2022-11-01 20:28 ` Sean Christopherson
2022-11-01 20:46 ` Oliver Upton
2022-11-01 21:07 ` Sean Christopherson [this message]
2022-10-27 22:22 ` [PATCH v3 10/15] KVM: arm64: Atomically update stage 2 leaf attributes in parallel walks Oliver Upton
2022-10-27 22:22 ` [PATCH v3 11/15] KVM: arm64: Split init and set for table PTE Oliver Upton
2022-10-27 22:22 ` [PATCH v3 12/15] KVM: arm64: Make block->table PTE changes parallel-aware Oliver Upton
2022-11-02 2:22 ` Ricardo Koller
2022-11-02 23:03 ` Oliver Upton
2022-10-27 22:23 ` [PATCH v3 13/15] KVM: arm64: Make leaf->leaf " Oliver Upton
2022-10-27 22:23 ` [PATCH v3 14/15] KVM: arm64: Make table->block " Oliver Upton
2022-10-27 22:23 ` [PATCH v3 15/15] KVM: arm64: Handle stage-2 faults in parallel Oliver Upton
2022-10-27 23:39 ` [PATCH v3 00/15] KVM: arm64: Parallel stage-2 fault handling Oliver Upton
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