linux-kernel - Re: [PATCH v2 00/13] KVM: Introduce KVM Userfault

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Message-ID: <aBqfsGtUjBUJudFB@google.com>
Date: Tue, 6 May 2025 16:48:00 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: James Houghton <jthoughton@...gle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, Jonathan Corbet <corbet@....net>, Marc Zyngier <maz@...nel.org>, 
	Oliver Upton <oliver.upton@...ux.dev>, 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 v2 00/13] KVM: Introduce KVM Userfault

On Thu, Jan 09, 2025, James Houghton wrote:
> James Houghton (13):
>   KVM: Add KVM_MEM_USERFAULT memslot flag and bitmap
>   KVM: Add KVM_MEMORY_EXIT_FLAG_USERFAULT
>   KVM: Allow late setting of KVM_MEM_USERFAULT on guest_memfd memslot
>   KVM: Advertise KVM_CAP_USERFAULT in KVM_CHECK_EXTENSION

Starting with some series-wide feedback, the granularity of these first few
patches is too fine.  I normally like to split things up, but honestly, this is
such a small feature that I don't see much point in separating the uAPI from the
infrastructure.

To avoid cyclical dependencies between common KVM and arch code, we can do all
the prep, but not fully enable+advertise support on any architecture until all
targeted architectures are fully ready.

In other words, I think we should squish these into one patch, minus this bit at
the very end of the series (spoiler alert):

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index ce7bf5de6d72..0106d6d461a3 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -1545,6 +1545,9 @@ static int check_memory_region_flags(struct kvm *kvm,
            !(mem->flags & KVM_MEM_GUEST_MEMFD))
                valid_flags |= KVM_MEM_READONLY;
 
+       if (IS_ENABLED(CONFIG_KVM_GENERIC_PAGE_FAULT))
+               valid_flags |= KVM_MEM_USERFAULT;
+
        if (mem->flags & ~valid_flags)
                return -EINVAL;
 
@@ -4824,6 +4827,9 @@ static int kvm_vm_ioctl_check_extension_generic(struct kvm *kvm, long arg)
        case KVM_CAP_CHECK_EXTENSION_VM:
        case KVM_CAP_ENABLE_CAP_VM:
        case KVM_CAP_HALT_POLL:
+#ifdef CONFIG_KVM_GENERIC_PAGE_FAULT
+       case KVM_CAP_USERFAULT:
+#endif
                return 1;
 #ifdef CONFIG_KVM_MMIO
        case KVM_CAP_COALESCED_MMIO: