linux-kernel - Re: [PATCH 07/11] KVM: guest_memfd: extract __kvm_gmem_get

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Message-ID: <ZimO7V2udx8KB2Li@google.com>
Date: Wed, 24 Apr 2024 15:59:57 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Paolo Bonzini <pbonzini@...hat.com>
Cc: linux-kernel@...r.kernel.org, kvm@...r.kernel.org, michael.roth@....com, 
	isaku.yamahata@...el.com
Subject: Re: [PATCH 07/11] KVM: guest_memfd: extract __kvm_gmem_get_pfn()

On Wed, Apr 24, 2024, Sean Christopherson wrote:
> On Thu, Apr 04, 2024, Paolo Bonzini wrote:
> > In preparation for adding a function that walks a set of pages
> > provided by userspace and populates them in a guest_memfd,
> > add a version of kvm_gmem_get_pfn() that has a "bool prepare"
> > argument and passes it down to kvm_gmem_get_folio().
> > 
> > Populating guest memory has to call repeatedly __kvm_gmem_get_pfn()
> > on the same file, so make the new function take struct file*.
> > 
> > Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> > ---
> >  virt/kvm/guest_memfd.c | 38 +++++++++++++++++++++++---------------
> >  1 file changed, 23 insertions(+), 15 deletions(-)
> > 
> > diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
> > index 486748e65f36..a537a7e63ab5 100644
> > --- a/virt/kvm/guest_memfd.c
> > +++ b/virt/kvm/guest_memfd.c
> > @@ -540,33 +540,29 @@ void kvm_gmem_unbind(struct kvm_memory_slot *slot)
> >  	fput(file);
> >  }
> >  
> > -int kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot,
> > -		     gfn_t gfn, kvm_pfn_t *pfn, int *max_order)
> > +static int __kvm_gmem_get_pfn(struct file *file, struct kvm_memory_slot *slot,
> > +		       gfn_t gfn, kvm_pfn_t *pfn, int *max_order, bool prepare)
> 
> I genuinely don't know what it means to "prepare" a guest_memfd.  I see it becomes
> 
> 	if (!prepare)
> 		fgp_flags |= FGP_CREAT_ONLY;
> 
> but I find the name "prepare" to be extremely unhelpful.

Ah, I'm blind.  Maybe "do_prepare", or "do_arch_prepare"?  To make it clear that
it's a command, not a description of the operation (which is how I first read it).

And I feel like overloading it to also mean FGP_CREAT_ONLY when _not_ preparing
the memory is odd.

	if (prepare) {
		int r =	kvm_gmem_prepare_folio(inode, index, folio);
		if (r < 0) {
			folio_unlock(folio);
			folio_put(folio);
			return ERR_PTR(r);
		}
	}

Instead of "prepare" as a command, would it make sense to describe the "populating"
case?  Because I think it's more intuitive that populating _needs_ to operate on
new, unprepared data.