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Message-ID: <aBqjV40pb-s9gvsz@google.com>
Date: Tue, 6 May 2025 17:03:35 -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 03/13] KVM: Allow late setting of KVM_MEM_USERFAULT on
guest_memfd memslot
On Thu, Jan 09, 2025, James Houghton wrote:
> Currently guest_memfd memslots can only be deleted. Slightly change the
> logic to allow KVM_MR_FLAGS_ONLY changes when the only flag being
> changed is KVM_MEM_USERFAULT.
>
> Signed-off-by: James Houghton <jthoughton@...gle.com>
> ---
> virt/kvm/kvm_main.c | 15 +++++++++++----
> 1 file changed, 11 insertions(+), 4 deletions(-)
>
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 4bceae6a6401..882c1f7b4aa8 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -2015,9 +2015,6 @@ int __kvm_set_memory_region(struct kvm *kvm,
> if ((kvm->nr_memslot_pages + npages) < kvm->nr_memslot_pages)
> return -EINVAL;
> } else { /* Modify an existing slot. */
> - /* Private memslots are immutable, they can only be deleted. */
> - if (mem->flags & KVM_MEM_GUEST_MEMFD)
> - return -EINVAL;
> if ((mem->userspace_addr != old->userspace_addr) ||
> (npages != old->npages) ||
> ((mem->flags ^ old->flags) & KVM_MEM_READONLY))
> @@ -2031,6 +2028,16 @@ int __kvm_set_memory_region(struct kvm *kvm,
> return 0;
> }
>
> + /*
> + * Except for being able to set KVM_MEM_USERFAULT, private memslots are
> + * immutable, they can only be deleted.
> + */
> + if (mem->flags & KVM_MEM_GUEST_MEMFD &&
> + !(change == KVM_MR_CREATE ||
> + (change == KVM_MR_FLAGS_ONLY &&
> + (mem->flags ^ old->flags) == KVM_MEM_USERFAULT)))
> + return -EINVAL;
Oof. I don't even want to decipher this. Let's just drop the blankent immutable
restriction, and simply say guest_memfd slots can't be MOVED. guest_memfd doesn't
support RO memslots (and never will), and doesn't support dirty logging, so this
is effectively dead code anyways. Then the diff is much nicer:
@@ -2012,14 +2027,15 @@ static int kvm_set_memory_region(struct kvm *kvm,
if ((kvm->nr_memslot_pages + npages) < kvm->nr_memslot_pages)
return -EINVAL;
} else { /* Modify an existing slot. */
- /* Private memslots are immutable, they can only be deleted. */
- if (mem->flags & KVM_MEM_GUEST_MEMFD)
- return -EINVAL;
if ((mem->userspace_addr != old->userspace_addr) ||
(npages != old->npages) ||
((mem->flags ^ old->flags) & KVM_MEM_READONLY))
return -EINVAL;
+ /* Moving a guest_memfd memslot isn't supported. */
+ if (base_gfn != old->base_gfn && mem->flags & KVM_MEM_GUEST_MEMFD)
+ return -EINVAL;
+
if (base_gfn != old->base_gfn)
change = KVM_MR_MOVE;
else if (mem->flags != old->flags)
> +
> if ((change == KVM_MR_CREATE || change == KVM_MR_MOVE) &&
> kvm_check_memslot_overlap(slots, id, base_gfn, base_gfn + npages))
> return -EEXIST;
> @@ -2046,7 +2053,7 @@ int __kvm_set_memory_region(struct kvm *kvm,
> new->npages = npages;
> new->flags = mem->flags;
> new->userspace_addr = mem->userspace_addr;
> - if (mem->flags & KVM_MEM_GUEST_MEMFD) {
> + if (mem->flags & KVM_MEM_GUEST_MEMFD && change == KVM_MR_CREATE) {
> r = kvm_gmem_bind(kvm, new, mem->guest_memfd, mem->guest_memfd_offset);
> if (r)
> goto out;
> --
> 2.47.1.613.gc27f4b7a9f-goog
>
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