Re: [PATCH v2 7/9] hostmem: Support in-place guest memfd to back a VM

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

On 12/13/2025 1:41 AM, Peter Xu wrote:
>>>> please add the following check as I commented in v1:
>>>>
>>>> 	if (!kvm_enabled()) {
>>>> 		error_setg(errp, "in-place guest-memfd requires KVM");
>>>> 		return false;
>>>> 	}
>>> IMHO it's redundant to set here, when kvm not enabled,
>>> kvm_create_guest_memfd() should be a stub.
>> No. The KVM stub is for the case where KVM is disable at compile time.
>>
>> The kvm_enabled() check here is for the case where users use different
>> accelerators other than KVM, e.g., -accel tcg.
> I thought the kvm-compiled case is already covered, at least the flags will
> be 0 here:
> 
>      if (!kvm_guest_memfd_supported) {
>          error_setg(errp, "KVM does not support guest_memfd");
>          return -1;
>      }
> 
> So I can change below stub patch into this one, would it look better (so
> that we'll provide explicit errors for all cases)?

It looks good to me.

> ===8<===
> commit 70012ceb70d3ffe624db33a8aeaaec581c7b4ccd
> Author: Peter Xu<peterx@redhat.com>
> Date:   Thu Dec 11 11:19:44 2025 -0500
> 
>      kvm: Provide explicit error for kvm_create_guest_memfd()
>      
>      So that there will be a verbal string returned when kvm not enabled, or kvm
>      not compiled.
>      
>      Signed-off-by: Peter Xu<peterx@redhat.com>
> ---
>   accel/kvm/kvm-all.c    | 5 +++++
>   accel/stubs/kvm-stub.c | 1 +
>   2 files changed, 6 insertions(+)
> 
> diff --git a/accel/kvm/kvm-all.c b/accel/kvm/kvm-all.c
> index 68d57c1af0..c32fbcf9cc 100644
> --- a/accel/kvm/kvm-all.c
> +++ b/accel/kvm/kvm-all.c
> @@ -4492,6 +4492,11 @@ int kvm_create_guest_memfd(uint64_t size, uint64_t flags, Error **errp)
>           .flags = flags,
>       };
>   
> +    if (!kvm_enabled()) {
> +        error_setg(errp, "guest-memfd requires KVM accelerator");
> +        return -1;
> +    }
> +
>       if (!kvm_guest_memfd_supported) {
>           error_setg(errp, "KVM does not support guest_memfd");
>           return -1;
> diff --git a/accel/stubs/kvm-stub.c b/accel/stubs/kvm-stub.c
> index 73f04eb589..01b1d6285e 100644
> --- a/accel/stubs/kvm-stub.c
> +++ b/accel/stubs/kvm-stub.c
> @@ -123,6 +123,7 @@ bool kvm_hwpoisoned_mem(void)
>   
>   int kvm_create_guest_memfd(uint64_t size, uint64_t flags, Error **errp)
>   {
> +    error_setg(errp, "KVM is not enabled");
>       return -ENOSYS;
>   }
> ===8<===
> 
> Thanks,