Re: [PATCH RFC v4 08/44] KVM: Introduce KVM_SET_MEMORY_ATTRIBUTES2

[Sean Christopherson <seanjc@google.com>]

On Tue, Mar 31, 2026, Michael Roth wrote:
> On Thu, Mar 26, 2026 at 03:24:17PM -0700, Ackerley Tng wrote:
> > Introduce a "version 2" of KVM_SET_MEMORY_ATTRIBUTES to support returning
> > information back to userspace.
> 
> Hi Ackerley,
> 
> Not trying to bikeshed below, but I'm working on getting related QEMU
> patches cleaned up to post soon and was working through some of the new
> uAPI bits, and plumbing some of these capabilities in seems a little
> awkward in a couple places so wondering if we should revisit how some of
> this API is defined...
> 
> > 
> > This new ioctl and structure will, in a later patch, be shared as a
> > guest_memfd ioctl, where the padding in the new kvm_memory_attributes2
> > structure will be for writing the response from the guest_memfd ioctl to
> > userspace.
> > 
> > A new ioctl is necessary for these reasons:
> > 
> > 1. KVM_SET_MEMORY_ATTRIBUTES is currently a write-only ioctl and does not
> >    allow userspace to read fields. There's nothing in code (yet?) that
> >    validates this, but using _IOWR for consistency would be prudent.
> > 
> > 2. KVM_SET_MEMORY_ATTRIBUTES, when used as a guest_memfd ioctl, will need
> >    an additional field to provide userspace with more error details.
> > 
> > Alternatively, a completely new ioctl could be defined, unrelated to
> > KVM_SET_MEMORY_ATTRIBUTES, but using the same ioctl number and struct for
> > the vm and guest_memfd ioctls streamlines the interface for userspace. In
> > addition, any memory attributes, implemented on the vm or guest_memfd
> > ioctl, can be easily shared with the other.
> > 
> > Add KVM_CAP_MEMORY_ATTRIBUTES2 to indicate that struct
> > kvm_memory_attributes2 exists and can be used either with
> > KVM_SET_MEMORY_ATTRIBUTES2 via the vm or guest_memfd ioctl.
> 
> The guest_memfd support for the KVM_SET_MEMORY_ATTRIBUTES2 ioctl isn't
> added until patch #10, and to scan for it you sort of need to infer it
> via KVM_CAP_GUEST_MEMFD_MEMORY_ATTRIBUTES reporting non-zero (i.e.
> KVM_MEMORY_ATTRIBUTE_PRIVATE), so it's confusing to state that
> KVM_CAP_MEMORY_ATTRIBUTES2 means you can use the struct via a guest_memfd
> ioctl.
> 
> I think the above is trying to simply say that the corresponding struct
> exists, and remain agnostic about how it can be used. But if that were
> the case, there would be no way to know when KVM_SET_MEMORY_ATTRIBUTES2 is
> available in the first place, so in the case of KVM ioctls at least,
> KVM_CAP_MEMORY_ATTRIBUTES2 is advertising both the struct and the ioctl,
> whereas for guest_memfd it's only advertising the struct and not saying
> anything about whether a similar gmem ioctl is available to use it.

+1 to everything Mike said.

> Instead, maybe they should both have the same semantics:
> 
>   KVM_CAP_MEMORY_ATTRIBUTES2: *SET_ATTRIBUTES* ioctl exists for KVM that utilizes
>     struct kvm_memory_attributes2
> 
>   KVM_CAP_GUEST_MEMFD_MEMORY_ATTRIBUTES: *SET_ATTRIBUTES* ioctl exists for
>     guest_memfd that utilizes struct kvm_memory_attributes2
> 
> In which case you would leave out any mention of guest_memfd here as far as
> the documentation does, and then in patch #10 you could modify it to be
> something like:
> 
>    4.145 KVM_SET_MEMORY_ATTRIBUTES2
>    ---------------------------------
> 
>   -:Capability: KVM_CAP_MEMORY_ATTRIBUTES2
>   +:Capability: KVM_CAP_MEMORY_ATTRIBUTES2, KVM_GUEST_MEMFD_CAP_MEMORY_ATTRIBUTES
>   -:Architectures: x86
>   +:Architectures: all
>   -:Type: vm ioctl
>   +:Type: vm, guest_memfd ioctl
>    :Parameters: struct kvm_memory_attributes2 (in/out)
>    :Returns: 0 on success, <0 on error

As discussed at PUCK, I think we should omit KVM_CAP_MEMORY_ATTRIBUTES2 and
vm-scoped support entirely until it's needed (which may be never).

> and *then* add in your mentions of how the usage/fields differ for
> guest_memfd/KVM_GUEST_MEMFD_CAP_MEMORY_ATTRIBUTES case vs. KVM ioctls.
> 
> This avoids needing to issue 2 checks for the guest_memfd variant vs. 1
> for KVM, but more importantly avoids subtle differences in how these
> similarly-named capabilities are used/documented that might cause
> unecessary confusion.