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 08/15] KVM: arm64: Protect stage-2 traversal with RCU
Date: Tue, 1 Nov 2022 20:22:15 +0000 [thread overview]
Message-ID: <Y2F/96RnvGPompIC@google.com> (raw)
In-Reply-To: <20221027221752.1683510-9-oliver.upton@linux.dev>
On Thu, Oct 27, 2022, Oliver Upton wrote:
> The use of RCU is necessary to safely change the stage-2 page tables in
> parallel.
RCU isn't strictly necessary, it's simply _a_ way to ensure readers have gone
away prior to freeing memory. E.g. another method (used on x86 at least), is to
disable IRQs when walking page tables without holding mmu_lock, and then requiring
paths that free page tables to effectively do an IPI shootdown of all CPUs that
are reading page tables.
And "safely change" is misleading. RCU doesn't allow safely _changing_ page
tables; if RCU did, then patch 10 wouldn't need to implement atomic updates.
Protecting page table walks with RCU is purely about ensuring readers have gone
away prior to freeing the backing memory, an entirely different mechanism is needed
to allow parallel updates, e.g. the CMPXCHG + KVM_INVALID_PTE_LOCKED approach to
ensure only one walker "owns" the detached page table.
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next prev parent reply other threads:[~2022-11-01 20:23 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 [this message]
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
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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