Re: [PATCH v12 48/84] KVM: Move x86's API to release a faultin page to common KVM

[Paolo Bonzini]

On 7/27/24 01:51, Sean Christopherson wrote:
> Move KVM x86's helper that "finishes" the faultin process to common KVM
> so that the logic can be shared across all architectures.  Note, not all
> architectures implement a fast page fault path, but the gist of the
> comment applies to all architectures.
>
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
>   arch/x86/kvm/mmu/mmu.c   | 24 ++----------------------
>   include/linux/kvm_host.h | 26 ++++++++++++++++++++++++++
>   2 files changed, 28 insertions(+), 22 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 95beb50748fc..2a0cfa225c8d 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -4323,28 +4323,8 @@ static u8 kvm_max_private_mapping_level(struct kvm *kvm, kvm_pfn_t pfn,
>   static void kvm_mmu_finish_page_fault(struct kvm_vcpu *vcpu,
>   				      struct kvm_page_fault *fault, int r)
>   {
> -	lockdep_assert_once(lockdep_is_held(&vcpu->kvm->mmu_lock) ||
> -			    r == RET_PF_RETRY);
> -
> -	if (!fault->refcounted_page)
> -		return;
> -
> -	/*
> -	 * If the page that KVM got from the *primary MMU* is writable, and KVM
> -	 * installed or reused a SPTE, mark the page/folio dirty.  Note, this
> -	 * may mark a folio dirty even if KVM created a read-only SPTE, e.g. if
> -	 * the GFN is write-protected.  Folios can't be safely marked dirty
> -	 * outside of mmu_lock as doing so could race with writeback on the
> -	 * folio.  As a result, KVM can't mark folios dirty in the fast page
> -	 * fault handler, and so KVM must (somewhat) speculatively mark the
> -	 * folio dirty if KVM could locklessly make the SPTE writable.
> -	 */
> -	if (r == RET_PF_RETRY)
> -		kvm_release_page_unused(fault->refcounted_page);
> -	else if (!fault->map_writable)
> -		kvm_release_page_clean(fault->refcounted_page);
> -	else
> -		kvm_release_page_dirty(fault->refcounted_page);
> +	kvm_release_faultin_page(vcpu->kvm, fault->refcounted_page,
> +				 r == RET_PF_RETRY, fault->map_writable);

Does it make sense to move RET_PF_* to common code, and avoid a bool 
argument here?

Paolo

>   }
>   
>   static int kvm_mmu_faultin_pfn_private(struct kvm_vcpu *vcpu,
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index 9d2a97eb30e4..91341cdc6562 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -1216,6 +1216,32 @@ static inline void kvm_release_page_unused(struct page *page)
>   void kvm_release_page_clean(struct page *page);
>   void kvm_release_page_dirty(struct page *page);
>   
> +static inline void kvm_release_faultin_page(struct kvm *kvm, struct page *page,
> +					    bool unused, bool dirty)
> +{
> +	lockdep_assert_once(lockdep_is_held(&kvm->mmu_lock) || unused);
> +
> +	if (!page)
> +		return;
> +
> +	/*
> +	 * If the page that KVM got from the *primary MMU* is writable, and KVM
> +	 * installed or reused a SPTE, mark the page/folio dirty.  Note, this
> +	 * may mark a folio dirty even if KVM created a read-only SPTE, e.g. if
> +	 * the GFN is write-protected.  Folios can't be safely marked dirty
> +	 * outside of mmu_lock as doing so could race with writeback on the
> +	 * folio.  As a result, KVM can't mark folios dirty in the fast page
> +	 * fault handler, and so KVM must (somewhat) speculatively mark the
> +	 * folio dirty if KVM could locklessly make the SPTE writable.
> +	 */
> +	if (unused)
> +		kvm_release_page_unused(page);
> +	else if (dirty)
> +		kvm_release_page_dirty(page);
> +	else
> +		kvm_release_page_clean(page);
> +}
> +
>   kvm_pfn_t kvm_lookup_pfn(struct kvm *kvm, gfn_t gfn);
>   kvm_pfn_t __kvm_faultin_pfn(const struct kvm_memory_slot *slot, gfn_t gfn,
>   			    unsigned int foll, bool *writable,