Re: [PATCH v12 41/84] KVM: x86/mmu: Mark pages/folios dirty at the origin of make_spte()

[Paolo Bonzini <pbonzini@redhat.com>]

On 7/27/24 01:51, Sean Christopherson wrote:
> Move the marking of folios dirty from make_spte() out to its callers,
> which have access to the _struct page_, not just the underlying pfn.
> Once all architectures follow suit, this will allow removing KVM's ugly
> hack where KVM elevates the refcount of VM_MIXEDMAP pfns that happen to
> be struct page memory.
> 
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
>   arch/x86/kvm/mmu/mmu.c         | 29 +++++++++++++++++++++++++++--
>   arch/x86/kvm/mmu/paging_tmpl.h |  5 +++++
>   arch/x86/kvm/mmu/spte.c        | 11 -----------
>   3 files changed, 32 insertions(+), 13 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 1cdd67707461..7e7b855ce1e1 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -2918,7 +2918,16 @@ static bool kvm_mmu_prefetch_sptes(struct kvm_vcpu *vcpu, gfn_t gfn, u64 *sptep,
>   	for (i = 0; i < nr_pages; i++, gfn++, sptep++) {
>   		mmu_set_spte(vcpu, slot, sptep, access, gfn,
>   			     page_to_pfn(pages[i]), NULL);
> -		kvm_release_page_clean(pages[i]);
> +
> +		/*
> +		 * KVM always prefetches writable pages from the primary MMU,
> +		 * and KVM can make its SPTE writable in the fast page, without

"with a fast page fault"

Paolo

> +		 * notifying the primary MMU.  Mark pages/folios dirty now to
> +		 * ensure file data is written back if it ends up being written
> +		 * by the guest.  Because KVM's prefetching GUPs writable PTEs,
> +		 * the probability of unnecessary writeback is extremely low.
> +		 */
> +		kvm_release_page_dirty(pages[i]);
>   	}
>   
>   	return true;
> @@ -4314,7 +4323,23 @@ 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)
>   {
> -	kvm_release_pfn_clean(fault->pfn);
> +	lockdep_assert_once(lockdep_is_held(&vcpu->kvm->mmu_lock) ||
> +			    r == RET_PF_RETRY);
> +
> +	/*
> +	 * 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 (!fault->map_writable || r == RET_PF_RETRY)
> +		kvm_release_pfn_clean(fault->pfn);
> +	else
> +		kvm_release_pfn_dirty(fault->pfn);
>   }
>   
>   static int kvm_mmu_faultin_pfn_private(struct kvm_vcpu *vcpu,
> diff --git a/arch/x86/kvm/mmu/paging_tmpl.h b/arch/x86/kvm/mmu/paging_tmpl.h
> index b6897916c76b..2e2d87a925ac 100644
> --- a/arch/x86/kvm/mmu/paging_tmpl.h
> +++ b/arch/x86/kvm/mmu/paging_tmpl.h
> @@ -953,6 +953,11 @@ static int FNAME(sync_spte)(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp, int
>   		  spte_to_pfn(spte), spte, true, false,
>   		  host_writable, &spte);
>   
> +	/*
> +	 * There is no need to mark the pfn dirty, as the new protections must
> +	 * be a subset of the old protections, i.e. synchronizing a SPTE cannot
> +	 * change the SPTE from read-only to writable.
> +	 */
>   	return mmu_spte_update(sptep, spte);
>   }
>   
> diff --git a/arch/x86/kvm/mmu/spte.c b/arch/x86/kvm/mmu/spte.c
> index 9b8795bd2f04..2c5650390d3b 100644
> --- a/arch/x86/kvm/mmu/spte.c
> +++ b/arch/x86/kvm/mmu/spte.c
> @@ -277,17 +277,6 @@ bool make_spte(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp,
>   		mark_page_dirty_in_slot(vcpu->kvm, slot, gfn);
>   	}
>   
> -	/*
> -	 * If the page that KVM got from the primary MMU is writable, i.e. if
> -	 * it's host-writable, mark the page/folio dirty.  As alluded to above,
> -	 * folios can't be safely marked dirty in the fast page fault handler,
> -	 * and so KVM must (somewhat) speculatively mark the folio dirty even
> -	 * though it isn't guaranteed to be written as KVM won't mark the folio
> -	 * dirty if/when the SPTE is made writable.
> -	 */
> -	if (host_writable)
> -		kvm_set_pfn_dirty(pfn);
> -
>   	*new_spte = spte;
>   	return wrprot;
>   }