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

[Fuad Tabba <tabba@google.com>]

Hi Shivank,

On Tue, 8 Apr 2025 at 13:04, Shivank Garg <shivankg@amd.com> wrote:
>
> 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()?

Makes sense.

Thanks!

/fuad

> > +     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