Subject: Re: [PATCH v7 4/8] KVM: x86/mmu: Migrate to __kvm_follow_pfn

From: Sean Christopherson <[email-masked]>
Date: Fri, 4 Aug 2023 15:30:24 -0700

On Wed, Jul 05, 2023, Yu Zhang wrote:
> On Tue, Jul 04, 2023 at 04:50:49PM +0900, David Stevens wrote:
> > From: David Stevens <[email-masked]>
> > 
> > Migrate from __gfn_to_pfn_memslot to __kvm_follow_pfn.

Please turn up your changelog verbosity from ~2 to ~8.  E.g. explain the transition
from async => FOLL_NOWAIT+KVM_PFN_ERR_NEEDS_IO, there's no reason to force readers
to suss that out on their own.

> > Signed-off-by: David Stevens <[email-masked]>
> > ---
> >  arch/x86/kvm/mmu/mmu.c | 35 +++++++++++++++++++++++++----------
> >  1 file changed, 25 insertions(+), 10 deletions(-)
> > 
> > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> > index ec169f5c7dce..e44ab512c3a1 100644
> > --- a/arch/x86/kvm/mmu/mmu.c
> > +++ b/arch/x86/kvm/mmu/mmu.c
> > @@ -4296,7 +4296,12 @@ void kvm_arch_async_page_ready(struct kvm_vcpu *vcpu, struct kvm_async_pf *work)
> >  static int __kvm_faultin_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
> >  {
> >  	struct kvm_memory_slot *slot = fault->slot;
> > -	bool async;
> > +	struct kvm_follow_pfn foll = {
> > +		.slot = slot,
> > +		.gfn = fault->gfn,
> > +		.flags = FOLL_GET | (fault->write ? FOLL_WRITE : 0),
> > +		.allow_write_mapping = true,
> > +	};
> >  
> >  	/*
> >  	 * Retry the page fault if the gfn hit a memslot that is being deleted
> > @@ -4325,12 +4330,14 @@ static int __kvm_faultin_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault
> >  			return RET_PF_EMULATE;
> >  	}
> >  
> > -	async = false;
> > -	fault->pfn = __gfn_to_pfn_memslot(slot, fault->gfn, false, false, &async,
> > -					  fault->write, &fault->map_writable,
> > -					  &fault->hva);
> > -	if (!async)
> > -		return RET_PF_CONTINUE; /* *pfn has correct page already */
> > +	foll.flags |= FOLL_NOWAIT;
> > +	fault->pfn = __kvm_follow_pfn(&foll);
> > +
> > +	if (!is_error_noslot_pfn(fault->pfn))
> > +		goto success;
> > +
> > +	if (fault->pfn != KVM_PFN_ERR_NEEDS_IO)
> > +		return RET_PF_CONTINUE;
> 
> IIUC, FOLL_NOWAIT is set only when we wanna an async fault. So
> KVM_PFN_ERR_NEEDS_IO may not be necessary? 

But FOLL_NOWAIT is set above.  This logic is essentially saying "bail immediately
if __gfn_to_pfn_memslot() returned a fatal error".

A commented would definitely be helpful though.  How about?

	/*
	 * If __kvm_follow_pfn() failed because I/O is needed to fault in the
	 * page, then either set up an asynchronous #PF to do the I/O, or if
	 * doing an async #PF isn't possible, retry __kvm_follow_pfn() with
	  I/O allowed. All other failures are fatal, i.e. retrying won't help.
	 */
	if (fault->pfn != KVM_PFN_ERR_NEEDS_IO)
		return RET_PF_CONTINUE;