From: Oliver Upton <oliver.upton@linux.dev>
To: Mingwei Zhang <mizhang@google.com>
Cc: Marek Szyprowski <m.szyprowski@samsung.com>,
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>,
Sean Christopherson <seanjc@google.com>,
kvmarm@lists.linux.dev
Subject: Re: [PATCH v5 08/14] KVM: arm64: Protect stage-2 traversal with RCU
Date: Mon, 5 Dec 2022 07:47:45 +0000 [thread overview]
Message-ID: <Y42iIXWbwxQ138fI@google.com> (raw)
In-Reply-To: <Y42G0c9yBk2KHc+g@google.com>
Hi Mingwei,
On Mon, Dec 05, 2022 at 05:51:13AM +0000, Mingwei Zhang wrote:
> On Mon, Nov 14, 2022, Oliver Upton wrote:
[...]
> > As hyp stage-1 is protected by a spinlock there is no actual need for
> > RCU in that case. I'll post something later on today that addresses the
> > issue.
> >
>
> For each stage-2 page table walk, KVM will use
> kvm_mmu_topup_memory_cache() before taking the mmu lock. This ensures
> whoever holding the mmu lock won't sleep. hyp walkers seems to
> miss this notion completely, whic makes me puzzeled. Using a spinlock
> only ensures functionality but seems quite inefficient if the one who
> holds the spinlock try to allocate pages and sleep...
You're probably confused by my mischaracterization in the above
paragraph. Hyp stage-1 walkers (outside of pKVM) are guarded with a
mutex and are perfectly able to sleep. The erroneous application of RCU
led to this path becoming non-sleepable, hence the bug.
pKVM's own hyp stage-1 walkers are guarded by a spinlock, but the memory
allocations come from its own allocator and there is no concept of a
scheduler at EL2.
> Why do we need an RCU lock here. Oh is it for batching?
We definitely don't need RCU here, thus the corrective measure was to
avoid RCU for exclusive table walks.
https://git.kernel.org/pub/scm/linux/kernel/git/kvmarm/kvmarm.git/commit/?h=next&id=b7833bf202e3068abb77c642a0843f696e9c8d38
--
Thanks,
Oliver
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next prev parent reply other threads:[~2022-12-05 7:49 UTC|newest]
Thread overview: 52+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-11-07 21:56 [PATCH v5 00/14] KVM: arm64: Parallel stage-2 fault handling Oliver Upton
2022-11-07 21:56 ` [PATCH v5 01/14] KVM: arm64: Combine visitor arguments into a context structure Oliver Upton
2022-11-09 22:23 ` Ben Gardon
2022-11-09 22:48 ` Oliver Upton
2022-11-10 0:23 ` Gavin Shan
2022-11-10 0:42 ` Oliver Upton
2022-11-10 3:40 ` Gavin Shan
2022-11-07 21:56 ` [PATCH v5 02/14] KVM: arm64: Stash observed pte value in visitor context Oliver Upton
2022-11-09 22:23 ` Ben Gardon
2022-11-10 4:55 ` Gavin Shan
2022-11-07 21:56 ` [PATCH v5 03/14] KVM: arm64: Pass mm_ops through the " Oliver Upton
2022-11-09 22:23 ` Ben Gardon
2022-11-10 5:22 ` Gavin Shan
2022-11-10 5:30 ` Gavin Shan
2022-11-07 21:56 ` [PATCH v5 04/14] KVM: arm64: Don't pass kvm_pgtable through kvm_pgtable_walk_data Oliver Upton
2022-11-09 22:23 ` Ben Gardon
2022-11-10 5:30 ` Gavin Shan
2022-11-10 5:38 ` Oliver Upton
2022-11-07 21:56 ` [PATCH v5 05/14] KVM: arm64: Add a helper to tear down unlinked stage-2 subtrees Oliver Upton
2022-11-09 22:23 ` Ben Gardon
2022-11-09 22:54 ` Oliver Upton
2022-11-07 21:56 ` [PATCH v5 06/14] KVM: arm64: Use an opaque type for pteps Oliver Upton
2022-11-09 22:23 ` Ben Gardon
2022-11-07 21:56 ` [PATCH v5 07/14] KVM: arm64: Tear down unlinked stage-2 subtree after break-before-make Oliver Upton
2022-11-09 22:24 ` Ben Gardon
2022-11-07 21:56 ` [PATCH v5 08/14] KVM: arm64: Protect stage-2 traversal with RCU Oliver Upton
2022-11-09 21:53 ` Sean Christopherson
2022-11-09 23:55 ` Oliver Upton
2022-11-15 18:47 ` Ricardo Koller
2022-11-15 18:57 ` Oliver Upton
2022-11-09 22:25 ` Ben Gardon
2022-11-10 13:34 ` Marc Zyngier
[not found] ` <CGME20221114142915eucas1p258f3ca2c536bde712c068e96851468fd@eucas1p2.samsung.com>
2022-11-14 14:29 ` Marek Szyprowski
2022-11-14 17:42 ` Oliver Upton
2022-12-05 5:51 ` Mingwei Zhang
2022-12-05 7:47 ` Oliver Upton [this message]
2022-11-07 21:56 ` [PATCH v5 09/14] KVM: arm64: Atomically update stage 2 leaf attributes in parallel walks Oliver Upton
2022-11-09 22:26 ` Ben Gardon
2022-11-09 22:42 ` Sean Christopherson
2022-11-09 23:00 ` Ben Gardon
2022-11-10 13:40 ` Marc Zyngier
2022-11-07 21:56 ` [PATCH v5 10/14] KVM: arm64: Split init and set for table PTE Oliver Upton
2022-11-09 22:26 ` Ben Gardon
2022-11-09 23:00 ` Oliver Upton
2022-11-07 21:58 ` [PATCH v5 11/14] KVM: arm64: Make block->table PTE changes parallel-aware Oliver Upton
2022-11-09 22:26 ` Ben Gardon
2022-11-09 23:03 ` Oliver Upton
2022-11-07 21:59 ` [PATCH v5 12/14] KVM: arm64: Make leaf->leaf " Oliver Upton
2022-11-09 22:26 ` Ben Gardon
2022-11-07 22:00 ` [PATCH v5 13/14] KVM: arm64: Make table->block " Oliver Upton
2022-11-07 22:00 ` [PATCH v5 14/14] KVM: arm64: Handle stage-2 faults in parallel Oliver Upton
2022-11-11 15:47 ` [PATCH v5 00/14] KVM: arm64: Parallel stage-2 fault handling Marc Zyngier
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