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

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Message-ID: <aFMWQ5_zMXGTCE98@linux.dev>
Date: Wed, 18 Jun 2025 12:40:51 -0700
From: Oliver Upton <oliver.upton@...ux.dev>
To: James Houghton <jthoughton@...gle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>,
	Sean Christopherson <seanjc@...gle.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 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@...gle.com>
> Signed-off-by: Sean Christopherson <seanjc@...gle.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