linux-kernel - Re: [PATCH v5 4/7] KVM: x86/mmu: Only grab RCU lock for nx hugepage recovery for TDP MMU

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Message-ID: <aIFIPm5zENeKlgkw@google.com>
Date: Wed, 23 Jul 2025 13:38:22 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: James Houghton <jthoughton@...gle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, Vipin Sharma <vipinsh@...gle.com>, 
	David Matlack <dmatlack@...gle.com>, kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 4/7] KVM: x86/mmu: Only grab RCU lock for nx hugepage
 recovery for TDP MMU

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.