Re: [PATCH v5 3/9] KVM: guest_memfd: Allow host to map guest_memfd() pages

[Fuad Tabba <tabba@google.com>]

Hi Kirill,

On Tue, 4 Mar 2025 at 08:58, Kirill A. Shutemov
<kirill.shutemov@linux.intel.com> wrote:
>
> On Mon, Mar 03, 2025 at 05:10:07PM +0000, 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>
> > ---
> >  include/linux/kvm_host.h |  11 ++++
> >  include/uapi/linux/kvm.h |   1 +
> >  virt/kvm/guest_memfd.c   | 105 +++++++++++++++++++++++++++++++++++++++
> >  virt/kvm/kvm_main.c      |   4 ++
> >  4 files changed, 121 insertions(+)
> >
> > diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> > index 7788e3625f6d..2d025b8ee20e 100644
> > --- a/include/linux/kvm_host.h
> > +++ b/include/linux/kvm_host.h
> > @@ -728,6 +728,17 @@ static inline bool kvm_arch_has_private_mem(struct kvm *kvm)
> >  }
> >  #endif
> >
> > +/*
> > + * Arch code must define kvm_arch_gmem_supports_shared_mem if support for
> > + * private memory is enabled and it supports in-place shared/private conversion.
> > + */
> > +#if !defined(kvm_arch_gmem_supports_shared_mem) && !IS_ENABLED(CONFIG_KVM_PRIVATE_MEM)
>
> Hm. Do we expect any caller for !CONFIG_KVM_PRIVATE_MEM?
>
> > +static inline bool kvm_arch_gmem_supports_shared_mem(struct kvm *kvm)
> > +{
> > +     return false;
> > +}
> > +#endif
> > +
> >  #ifndef kvm_arch_has_readonly_mem
> >  static inline bool kvm_arch_has_readonly_mem(struct kvm *kvm)
> >  {
>
> ...
>
> > diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
> > index b2aa6bf24d3a..4291956b51ae 100644
> > --- a/virt/kvm/guest_memfd.c
> > +++ b/virt/kvm/guest_memfd.c
> > @@ -312,7 +312,112 @@ static pgoff_t kvm_gmem_get_index(struct kvm_memory_slot *slot, gfn_t gfn)
> >       return gfn - slot->base_gfn + slot->gmem.pgoff;
> >  }
> >
> > +#ifdef CONFIG_KVM_GMEM_SHARED_MEM
> > +static bool kvm_gmem_offset_is_shared(struct file *file, pgoff_t index)
> > +{
> > +     struct kvm_gmem *gmem = file->private_data;
> > +
> > +     /* For now, VMs that support shared memory share all their memory. */
> > +     return kvm_arch_gmem_supports_shared_mem(gmem->kvm);
> > +}
> > +
> > +static vm_fault_t kvm_gmem_fault(struct vm_fault *vmf)
> > +{
> > +     struct inode *inode = file_inode(vmf->vma->vm_file);
> > +     struct folio *folio;
> > +     vm_fault_t ret = VM_FAULT_LOCKED;
> > +
> > +     filemap_invalidate_lock_shared(inode->i_mapping);
> > +
> > +     folio = kvm_gmem_get_folio(inode, vmf->pgoff);
> > +     if (IS_ERR(folio)) {
> > +             switch (PTR_ERR(folio)) {
> > +             case -EAGAIN:
> > +                     ret = VM_FAULT_RETRY;
> > +                     break;
> > +             case -ENOMEM:
> > +                     ret = VM_FAULT_OOM;
> > +                     break;
> > +             default:
> > +                     ret = VM_FAULT_SIGBUS;
> > +                     break;
> > +             }
> > +             goto out_filemap;
> > +     }
> > +
> > +     if (folio_test_hwpoison(folio)) {
> > +             ret = VM_FAULT_HWPOISON;
> > +             goto out_folio;
> > +     }
> > +
> > +     /* Must be called with folio lock held, i.e., after kvm_gmem_get_folio() */
>
> If this is a requirement, it would be cleaner to rename the function and
> pass down the folio and check the lock state inside.

Will do.

Thanks,
/fuad

> --
>   Kiryl Shutsemau / Kirill A. Shutemov