linux-kernel - Re: [PATCH 1/3] KVM: x86/mmu: remove unnecessary "bool shared" argument from functions

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Message-ID: <ZRb3LX-tMKtycBjm@google.com>
Date:   Fri, 29 Sep 2023 09:11:25 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH 1/3] KVM: x86/mmu: remove unnecessary "bool shared"
 argument from functions

On Thu, Sep 28, 2023, Paolo Bonzini wrote:
> Neither tdp_mmu_next_root nor kvm_tdp_mmu_put_root need to know
> if the lock is taken for read or write.  Either way, protection
> is achieved via RCU and tdp_mmu_pages_lock.  Remove the argument
> and just assert that the lock is taken.
> 
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> ---
> +void kvm_tdp_mmu_put_root(struct kvm *kvm, struct kvm_mmu_page *root)
>  {
> -	kvm_lockdep_assert_mmu_lock_held(kvm, shared);
> +	/*
> +	 * Either read or write is okay, but the lock is needed because
> +	 * writers might not take tdp_mmu_pages_lock.
> +	 */

Nit, I'd prefer to say mmu_lock instead of "the lock", and be very explicit about
writers not needing to take tdp_mmu_pages_lock, e.g.

	/*
	 * Either read or write is okay, but mmu_lock must be held as writers
	 * are not required to take tdp_mmu_pages_lock.
	 */


> +	lockdep_assert_held(&kvm->mmu_lock);
>  
>  	if (!refcount_dec_and_test(&root->tdp_mmu_root_count))
>  		return;