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

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

On Wed, Jun 18, 2025 at 04:24:13AM +0000, James Houghton wrote:
> KVM Userfault consists of a bitmap in userspace that describes which
> pages the user wants exits on (when KVM_MEM_USERFAULT is enabled). To
> get those exits, the memslot where KVM_MEM_USERFAULT is being enabled
> must drop (at least) all of the translations that the bitmap says should
> generate faults. Today, simply drop all translations for the memslot. Do
> so with a new arch interface, kvm_arch_userfault_enabled(), which can be
> specialized in the future by any architecture for which optimizations
> make sense.
> 
> Make some changes to kvm_set_memory_region() to support setting
> KVM_MEM_USERFAULT on KVM_MEM_GUEST_MEMFD memslots, including relaxing
> the retrictions on guest_memfd memslots from only deletion to no moving.
> 
> Signed-off-by: James Houghton <jthoughton@google.com>
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---

> +#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.

> +{
> +	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;
> +

I see that the documentation suggests userspace perform a store-release
to update the bitmap. That's the right idea but we need a load-acquire
on the consumer side for that to do something meaningful.

> +	if (!test_bit(offset % BITS_PER_LONG, &chunk))
> +		return false;
> +
> +	kvm_prepare_memory_fault_exit(vcpu, fault);
> +	vcpu->run->memory_fault.flags |= KVM_MEMORY_EXIT_FLAG_USERFAULT;
> +	return true;
> +}
> +#endif
> +
>  int __attribute__((weak)) kvm_vm_ioctl_enable_cap(struct kvm *kvm,
>  						  struct kvm_enable_cap *cap)
>  {
> -- 
> 2.50.0.rc2.692.g299adb8693-goog
>

Thanks,
Oliver