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

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Message-ID: <aFNBCaLEdABfybmd@linux.dev>
Date: Wed, 18 Jun 2025 15:43:21 -0700
From: Oliver Upton <oliver.upton@...ux.dev>
To: Sean Christopherson <seanjc@...gle.com>
Cc: James Houghton <jthoughton@...gle.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Jonathan Corbet <corbet@....net>, Marc Zyngier <maz@...nel.org>,
	Yan Zhao <yan.y.zhao@...el.com>,
	Nikita Kalyazin <kalyazin@...zon.com>,
	Anish Moorthy <amoorthy@...gle.com>,
	Peter Gonda <pgonda@...gle.com>, Peter Xu <peterx@...hat.com>,
	David Matlack <dmatlack@...gle.com>, wei.w.wang@...el.com,
	kvm@...r.kernel.org, linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	kvmarm@...ts.linux.dev
Subject: Re: [PATCH v3 04/15] KVM: Add common infrastructure for KVM
 Userfaults

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@...gle.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