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

[Ackerley Tng <ackerleytng@google.com>]

Shivank Garg <shivankg@amd.com> writes:

> 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()?
>

Could you explain a little more about this being directly visible to
userspace?

IIUC generic_fillattr(), which guest_memfd uses, will fill stat->atime
from the inode's atime. file_accessed() will update atime and so this
should be userspace accessible. (Unless I missed something along the way
that blocks the update)

>> +	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