Subject: Re: [RFC PATCH v11 12/29] KVM: Add KVM_CREATE_GUEST_MEMFD ioctl() for guest-specific backing memory

From: Sean Christopherson <[email-masked]>
Date: Thu, 20 Jul 2023 08:14:47 -0700

On Thu, Jul 20, 2023, Xiaoyao Li wrote:
> On 7/19/2023 7:44 AM, Sean Christopherson wrote:
> > @@ -5134,6 +5167,16 @@ static long kvm_vm_ioctl(struct file *filp,
> >   	case KVM_GET_STATS_FD:
> >   		r = kvm_vm_ioctl_get_stats_fd(kvm);
> >   		break;
> > +	case KVM_CREATE_GUEST_MEMFD: {
> > +		struct kvm_create_guest_memfd guest_memfd;
> > +
> > +		r = -EFAULT;
> > +		if (copy_from_user(&guest_memfd, argp, sizeof(guest_memfd)))
> > +			goto out;
> > +
> > +		r = kvm_gmem_create(kvm, &guest_memfd);
> > +		break;
> > +	}
> 
> Does it need a new CAP to indicate the support of guest_memfd?

Yeah, I meant to add that to the TODO list and forgot (obviously).

> This is patch series introduces 3 new CAPs and it seems any one of them can
> serve as the indicator of guest_memfd.
> 
> +#define KVM_CAP_USER_MEMORY2 230
> +#define KVM_CAP_MEMORY_ATTRIBUTES 231
> +#define KVM_CAP_VM_TYPES 232

The number of new caps being added is the main why I didn't just add another one.
On the other hand, we have room for a few billion caps, so one more isn't a big
deal.  So yeah, KVM_CAP_GUEST_MEMFD is probably the way to go.

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