Re: [PATCH v7 3/9] KVM: guest_memfd: Allow host to map guest_memfd() pages

[Shivank Garg <shivankg@amd.com>]

Hi Fuad,

On 3/18/2025 9:48 PM, Fuad Tabba wrote:
> Add support for mmap() and fault() for guest_memfd backed memory
> in the host for VMs that support in-place conversion between
> shared and private. To that end, this patch adds the ability to
> check whether the VM type supports in-place conversion, and only
> allows mapping its memory if that's the case.
> 
> Also add the KVM capability KVM_CAP_GMEM_SHARED_MEM, which
> indicates that the VM supports shared memory in guest_memfd, or
> that the host can create VMs that support shared memory.
> Supporting shared memory implies that memory can be mapped when
> shared with the host.
> 
> This is controlled by the KVM_GMEM_SHARED_MEM configuration
> option.
> 
> Signed-off-by: Fuad Tabba <tabba@google.com>

...
...
> +
> +static int kvm_gmem_mmap(struct file *file, struct vm_area_struct *vma)
> +{
> +	struct kvm_gmem *gmem = file->private_data;
> +
> +	if (!kvm_arch_gmem_supports_shared_mem(gmem->kvm))
> +		return -ENODEV;
> +
> +	if ((vma->vm_flags & (VM_SHARED | VM_MAYSHARE)) !=
> +	    (VM_SHARED | VM_MAYSHARE)) {
> +		return -EINVAL;
> +	}
> +
> +	file_accessed(file);

As it is not directly visible to userspace, do we need to update the
file's access time via file_accessed()?

> +	vm_flags_set(vma, VM_DONTDUMP);
> +	vma->vm_ops = &kvm_gmem_vm_ops;
> +
> +	return 0;
> +}
> +#else
> +#define kvm_gmem_mmap NULL
> +#endif /* CONFIG_KVM_GMEM_SHARED_MEM */
> +
>  static struct file_operations kvm_gmem_fops = {
> +	.mmap		= kvm_gmem_mmap,
>  	.open		= generic_file_open,
>  	.release	= kvm_gmem_release,
>  	.fallocate	= kvm_gmem_fallocate,

Thanks,
Shivank