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

From: Sean Christopherson <[email-masked]>
Date: Tue, 30 Jul 2024 12:15:38 -0700

On Tue, Jul 30, 2024, Paolo Bonzini wrote:
> 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 <[email-masked]>
> > ---
> >   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?

After this series, probably?  Especially if/when we make "struct kvm_page_fault"
a common structure and converge all arch code.  In this series, definitely not,
as it would require even more patches to convert other architectures, and it's
not clear that it would be a net win, at least not without even more massaging.