Re: [PATCH v8 17/46] KVM: guest_memfd: Advertise KVM_SET_MEMORY_ATTRIBUTES2 ioctl

[Binbin Wu]

On 6/19/2026 8:31 AM, Ackerley Tng via B4 Relay wrote:
> From: Ackerley Tng <ackerleytng@xxxxxxxxxx>
> 
> Introduce KVM_CAP_GUEST_MEMFD_MEMORY_ATTRIBUTES to advertise the
> availability of the KVM_SET_MEMORY_ATTRIBUTES2 ioctl.
> 
> KVM_SET_MEMORY_ATTRIBUTES2 is a guest_memfd-scoped version of the existing
> KVM_SET_MEMORY_ATTRIBUTES VM ioctl. It allows userspace to manage memory
> attributes, such as KVM_MEMORY_ATTRIBUTE_PRIVATE, directly on a guest_memfd
> file descriptor.
> 
> This new version uses struct kvm_memory_attributes2, which adds an
> error_offset field to the output. This allows KVM to return the specific
> offset that triggered an error, which is especially useful for handling
> EAGAIN results caused by transient page reference counts during attribute
> conversions.
> 
> Update the KVM API documentation to define the new ioctl and its behavior,
> and add the necessary UAPI definitions and capability checks.
> 
> Suggested-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> Suggested-by: Michael Roth <michael.roth@xxxxxxx>
> Signed-off-by: Ackerley Tng <ackerleytng@xxxxxxxxxx>

Reviewed-by: Binbin Wu <binbin.wu@xxxxxxxxxxxxxxx>

Two nits below.


>  
> +4.145 KVM_SET_MEMORY_ATTRIBUTES2
> +---------------------------------
> +
> +:Capability: KVM_CAP_GUEST_MEMFD_MEMORY_ATTRIBUTES
> +:Architectures: all
> +:Type: guest_memfd ioctl
> +:Parameters: struct kvm_memory_attributes2 (in/out)
> +:Returns: 0 on success, <0 on error
> +
> +Errors:
> +
> +  ========== ===============================================================
> +  EINVAL     The specified `offset` or `size` were invalid (e.g. not
                                                   ^
                                                 was
 > +             page aligned, causes an overflow, or size is zero).
> +  EFAULT     The parameter address was invalid.
> +  EAGAIN     Some page within requested range had unexpected refcounts. The
> +             offset of the page will be returned in `error_offset`.
> +  ENOMEM     Ran out of memory trying to track private/shared state
> +  ========== ===============================================================

[...]

> +
> +Set attributes for a range of offsets within a guest_memfd to
> +KVM_MEMORY_ATTRIBUTE_PRIVATE to limit the specified guest_memfd backed
> +memory range for guest_use. Even if KVM_CAP_GUEST_MEMFD_MMAP is
                         ^
                    guest use

> +supported, after a successful call to set
> +KVM_MEMORY_ATTRIBUTE_PRIVATE, the requested range will not be mappable
> +into host userspace and will only be mappable by the guest.
> +