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

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Message-ID: <aFMh51vXbTNCf9mv@google.com>
Date: Wed, 18 Jun 2025 13:33:17 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Oliver Upton <oliver.upton@...ux.dev>
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, 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;


> > +{
> > +	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;

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