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

[Sean Christopherson]

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.