Re: [PATCH v3 04/15] KVM: Add common infrastructure for KVM Userfaults

[Oliver Upton <oliver.upton@linux.dev>]

On Wed, Jun 18, 2025 at 01:33:17PM -0700, Sean Christopherson wrote:
> On Wed, Jun 18, 2025, Oliver Upton wrote:
> > > Signed-off-by: Sean Christopherson <seanjc@google.com>
> 
> No need for my SoB.
> 
> > > +#ifdef CONFIG_KVM_GENERIC_PAGE_FAULT
> > > +bool kvm_do_userfault(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
> > 
> > The polarity of the return here feels weird. If we want a value of 0 to
> > indicate success then int is a better return type.
> 
> The boolean is my fault/suggestion.  My thinking is that it would make the callers
> more intuitive, e.g. so that this reads "if do userfault, then exit to userspace
> with -EFAULT".
> 
> 	if (kvm_do_userfault(vcpu, fault))
> 		return -EFAULT;

Agreed, this reads correctly. My only issue is that when I read the
function signature, "bool" is usually wired the other way around.

> > > +{
> > > +	struct kvm_memory_slot *slot = fault->slot;
> > > +	unsigned long __user *user_chunk;
> > > +	unsigned long chunk;
> > > +	gfn_t offset;
> > > +
> > > +	if (!kvm_is_userfault_memslot(slot))
> > > +		return false;
> > > +
> > > +	offset = fault->gfn - slot->base_gfn;
> > > +	user_chunk = slot->userfault_bitmap + (offset / BITS_PER_LONG);
> > > +
> > > +	if (__get_user(chunk, user_chunk))
> > > +		return true;
> 
> And this path is other motiviation for returning a boolean.  To me, return "success"
> when a uaccess fails looks all kinds of wrong:
> 
> 	if (__get_user(chunk, user_chunk))
> 		return 0;

Yeah, that's gross. Although I would imagine we want to express
"failure" here, game over, out to userspace for resolution. So maybe:

	if (__get_user(chunk, user_chunk))
		return -EFAULT;

> That said, I don't have a super strong preference; normally I'm fanatical about
> not returning booleans.  :-D

+1, it isn't _that_ big of a deal, just noticed it as part of review.

Thanks,
Oliver