Hi Kirill,
On Tue, 4 Mar 2025 at 08:58, Kirill A. Shutemov
<kirill.shutemov@xxxxxxxxxxxxxxx> 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@xxxxxxxxxx>
> > ---
> > 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
