Re: [PATCH v1 03/44] KVM: arm64: Move setting the page as dirty out of the critical section

[Marc Zyngier <maz@kernel.org>]

On Wed, 27 Mar 2024 17:34:50 +0000,
Fuad Tabba <tabba@google.com> wrote:
> 
> Move the unlock earlier in user_mem_abort() to shorten the
> critical section. This also helps for future refactoring and
> reuse of similar code.
> 
> Signed-off-by: Fuad Tabba <tabba@google.com>
> ---
>  arch/arm64/kvm/mmu.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> index 18680771cdb0..3afc42d8833e 100644
> --- a/arch/arm64/kvm/mmu.c
> +++ b/arch/arm64/kvm/mmu.c
> @@ -1522,8 +1522,10 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>  
>  	read_lock(&kvm->mmu_lock);
>  	pgt = vcpu->arch.hw_mmu->pgt;
> -	if (mmu_invalidate_retry(kvm, mmu_seq))
> +	if (mmu_invalidate_retry(kvm, mmu_seq)) {
> +		ret = -EAGAIN;
>  		goto out_unlock;
> +	}
>  
>  	/*
>  	 * If we are not forced to use page mapping, check if we are
> @@ -1581,6 +1583,8 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>  					     memcache,
>  					     KVM_PGTABLE_WALK_HANDLE_FAULT |
>  					     KVM_PGTABLE_WALK_SHARED);
> +out_unlock:
> +	read_unlock(&kvm->mmu_lock);
>  
>  	/* Mark the page dirty only if the fault is handled successfully */
>  	if (writable && !ret) {
> @@ -1588,8 +1592,6 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>  		mark_page_dirty_in_slot(kvm, memslot, gfn);
>  	}
>  
> -out_unlock:
> -	read_unlock(&kvm->mmu_lock);
>  	kvm_release_pfn_clean(pfn);
>  	return ret != -EAGAIN ? ret : 0;
>  }

It now means that things such as marking a page dirty happens outside
of the lock, which may interact with the dirty log/bitmap stuff.

Can you elaborate on *why* this is correct?

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.