Re: [PATCH v2 2/9] kvm: Detect guest-memfd flags supported

[Xiaoyao Li <xiaoyao.li@intel.com>]

On 11/20/2025 1:29 AM, Peter Xu wrote:
> Detect supported guest-memfd flags by the current kernel, and reject
> creations of guest-memfd using invalid flags.  When the cap isn't
> available, then no flag is supported.
> 
> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---
>   accel/kvm/kvm-all.c | 11 +++++++++++
>   1 file changed, 11 insertions(+)
> 
> diff --git a/accel/kvm/kvm-all.c b/accel/kvm/kvm-all.c
> index 96c194ce54..f477014126 100644
> --- a/accel/kvm/kvm-all.c
> +++ b/accel/kvm/kvm-all.c
> @@ -108,6 +108,7 @@ static int kvm_sstep_flags;
>   static bool kvm_immediate_exit;
>   static uint64_t kvm_supported_memory_attributes;
>   static bool kvm_guest_memfd_supported;
> +static uint64_t kvm_guest_memfd_flags_supported;
>   static hwaddr kvm_max_slot_size = ~0;
>   
>   static const KVMCapabilityInfo kvm_required_capabilites[] = {
> @@ -2787,6 +2788,10 @@ static int kvm_init(AccelState *as, MachineState *ms)
>       kvm_guest_memfd_supported =
>           kvm_vm_check_extension(s, KVM_CAP_GUEST_MEMFD) &&
>           kvm_vm_check_extension(s, KVM_CAP_USER_MEMORY2);
> +
> +    ret = kvm_vm_check_extension(s, KVM_CAP_GUEST_MEMFD_FLAGS);
> +    kvm_guest_memfd_flags_supported = ret > 0 ? ret : 0;
> +
>       kvm_pre_fault_memory_supported = kvm_vm_check_extension(s, KVM_CAP_PRE_FAULT_MEMORY);
>   
>       if (s->kernel_irqchip_split == ON_OFF_AUTO_AUTO) {
> @@ -4492,6 +4497,12 @@ int kvm_create_guest_memfd(uint64_t size, uint64_t flags, Error **errp)
>           return -1;
>       }
>   
> +    if (flags & ~kvm_guest_memfd_flags_supported) {
> +        error_setg(errp, "KVM does not support guest-memfd flag: 0x%"PRIx64,

I'm thinking if need to add "for current VM" like:

	KVM does not support guest-memfd flag: xxx for current VM

because kvm_guest_memfd_flags_supported is got from VM-scope CAP and 
varies for different VM types.

> +                   flags & ~kvm_guest_memfd_flags_supported);
> +        return -1;
> +    }
> +
>       fd = kvm_vm_ioctl(kvm_state, KVM_CREATE_GUEST_MEMFD, &guest_memfd);
>       if (fd < 0) {
>           error_setg_errno(errp, errno, "Error creating KVM guest_memfd");