From: Sean Christopherson <seanjc@google.com>
To: James Houghton <jthoughton@google.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>,
Vipin Sharma <vipinsh@google.com>,
David Matlack <dmatlack@google.com>,
kvm@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v5 4/7] KVM: x86/mmu: Only grab RCU lock for nx hugepage recovery for TDP MMU
Date: Wed, 23 Jul 2025 13:38:22 -0700 [thread overview]
Message-ID: <aIFIPm5zENeKlgkw@google.com> (raw)
In-Reply-To: <20250707224720.4016504-5-jthoughton@google.com>
On Mon, Jul 07, 2025, James Houghton wrote:
> Now that we have separate paths for the TDP MMU, it is trivial to only
> grab rcu_read_lock() for the TDP MMU case.
Yeah, but it's also a largely pointless change. For the overwhelming majority of
deployments, rcu_read_{un}lock() does literally nothing. And when it does do
something, the cost is a single atomic.
I'm leaning quite strongly toward skipping this patch, as I find the code to be
much more readable if KVM grabs RCU unconditionally.
> We do not need to grab it for the shadow MMU, as pages are not RCU-freed in
> that case.
next prev parent reply other threads:[~2025-07-23 20:38 UTC|newest]
Thread overview: 29+ messages / expand[flat|nested] mbox.gz Atom feed top
2025-07-07 22:47 [PATCH v5 0/7] KVM: x86/mmu: Run TDP MMU NX huge page recovery under MMU read lock James Houghton
2025-07-07 22:47 ` [PATCH v5 1/7] KVM: x86/mmu: Track TDP MMU NX huge pages separately James Houghton
2025-08-19 17:57 ` Sean Christopherson
2025-07-07 22:47 ` [PATCH v5 2/7] KVM: x86/mmu: Rename kvm_tdp_mmu_zap_sp() to better indicate its purpose James Houghton
2025-07-07 22:47 ` [PATCH v5 3/7] KVM: x86/mmu: Recover TDP MMU NX huge pages using MMU read lock James Houghton
2025-07-23 20:34 ` Sean Christopherson
2025-07-28 18:07 ` James Houghton
2025-07-28 18:17 ` David Matlack
2025-07-28 21:38 ` Sean Christopherson
2025-07-28 21:48 ` James Houghton
2025-08-01 18:17 ` David Matlack
2025-08-01 22:00 ` Sean Christopherson
2025-08-12 19:21 ` David Matlack
2025-07-07 22:47 ` [PATCH v5 4/7] KVM: x86/mmu: Only grab RCU lock for nx hugepage recovery for TDP MMU James Houghton
2025-07-23 20:38 ` Sean Christopherson [this message]
2025-07-28 17:51 ` James Houghton
2025-07-07 22:47 ` [PATCH v5 5/7] KVM: selftests: Introduce a selftest to measure execution performance James Houghton
2025-07-23 20:50 ` Sean Christopherson
2025-07-29 0:18 ` James Houghton
2025-07-07 22:47 ` [PATCH v5 6/7] KVM: selftests: Provide extra mmap flags in vm_mem_add() James Houghton
2025-07-07 22:47 ` [PATCH v5 7/7] KVM: selftests: Add an NX huge pages jitter test James Houghton
2025-07-23 21:04 ` Sean Christopherson
2025-07-28 18:40 ` James Houghton
2025-08-01 14:11 ` Sean Christopherson
2025-08-01 18:45 ` James Houghton
2025-08-01 22:30 ` Sean Christopherson
2025-07-23 20:44 ` [PATCH v5 0/7] KVM: x86/mmu: Run TDP MMU NX huge page recovery under MMU read lock Sean Christopherson
2025-07-29 0:19 ` James Houghton
2025-08-19 23:12 ` Sean Christopherson
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