On 6/11/2025 7:03 PM, Fuad Tabba wrote:
> This patch enables support for shared memory in guest_memfd, including
> mapping that memory from host userspace.
>
> This functionality is gated by the KVM_GMEM_SHARED_MEM Kconfig option,
> and enabled for a given instance by the GUEST_MEMFD_FLAG_SUPPORT_SHARED
> flag at creation time.
>
> Reviewed-by: Gavin Shan <gshan@xxxxxxxxxx>
> Acked-by: David Hildenbrand <david@xxxxxxxxxx>
> Co-developed-by: Ackerley Tng <ackerleytng@xxxxxxxxxx>
> Signed-off-by: Ackerley Tng <ackerleytng@xxxxxxxxxx>
> Signed-off-by: Fuad Tabba <tabba@xxxxxxxxxx>
> ---
> include/linux/kvm_host.h | 13 +++++++
> include/uapi/linux/kvm.h | 1 +
> virt/kvm/Kconfig | 4 +++
> virt/kvm/guest_memfd.c | 73 ++++++++++++++++++++++++++++++++++++++++
> 4 files changed, 91 insertions(+)
>
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index 9a6712151a74..6b63556ca150 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -729,6 +729,19 @@ static inline bool kvm_arch_supports_gmem(struct kvm *kvm)
> }
> #endif
>
> +/*
> + * Returns true if this VM supports shared mem in guest_memfd.
> + *
> + * Arch code must define kvm_arch_supports_gmem_shared_mem if support for
> + * guest_memfd is enabled.
> + */
> +#if !defined(kvm_arch_supports_gmem_shared_mem)
> +static inline bool kvm_arch_supports_gmem_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/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> index d00b85cb168c..cb19150fd595 100644
> --- a/include/uapi/linux/kvm.h
> +++ b/include/uapi/linux/kvm.h
> @@ -1570,6 +1570,7 @@ struct kvm_memory_attributes {
> #define KVM_MEMORY_ATTRIBUTE_PRIVATE (1ULL << 3)
>
> #define KVM_CREATE_GUEST_MEMFD _IOWR(KVMIO, 0xd4, struct kvm_create_guest_memfd)
> +#define GUEST_MEMFD_FLAG_SUPPORT_SHARED (1ULL << 0)
>
> struct kvm_create_guest_memfd {
> __u64 size;
> diff --git a/virt/kvm/Kconfig b/virt/kvm/Kconfig
> index 559c93ad90be..e90884f74404 100644
> --- a/virt/kvm/Kconfig
> +++ b/virt/kvm/Kconfig
> @@ -128,3 +128,7 @@ config HAVE_KVM_ARCH_GMEM_PREPARE
> config HAVE_KVM_ARCH_GMEM_INVALIDATE
> bool
> depends on KVM_GMEM
> +
> +config KVM_GMEM_SHARED_MEM
> + select KVM_GMEM
> + bool
> diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
> index 6db515833f61..06616b6b493b 100644
> --- a/virt/kvm/guest_memfd.c
> +++ b/virt/kvm/guest_memfd.c
> @@ -312,7 +312,77 @@ static pgoff_t kvm_gmem_get_index(struct kvm_memory_slot *slot, gfn_t gfn)
> return gfn - slot->base_gfn + slot->gmem.pgoff;
> }
>
> +static bool kvm_gmem_supports_shared(struct inode *inode)
> +{
> + const u64 flags = (u64)inode->i_private;
> +
> + if (!IS_ENABLED(CONFIG_KVM_GMEM_SHARED_MEM))
> + return false;
> +
> + return flags & GUEST_MEMFD_FLAG_SUPPORT_SHARED;
> +}
> +
> +static vm_fault_t kvm_gmem_fault_shared(struct vm_fault *vmf)
> +{
> + struct inode *inode = file_inode(vmf->vma->vm_file);
> + struct folio *folio;
> + vm_fault_t ret = VM_FAULT_LOCKED;
> +
> + if (((loff_t)vmf->pgoff << PAGE_SHIFT) >= i_size_read(inode))
> + return VM_FAULT_SIGBUS;
> +
> + folio = kvm_gmem_get_folio(inode, vmf->pgoff);
> + if (IS_ERR(folio)) {
> + int err = PTR_ERR(folio);
> +
> + if (err == -EAGAIN)
> + return VM_FAULT_RETRY;
> +
> + return vmf_error(err);
> + }
> +
> + if (WARN_ON_ONCE(folio_test_large(folio))) {
> + ret = VM_FAULT_SIGBUS;
> + goto out_folio;
> + }
> +
> + if (!folio_test_uptodate(folio)) {
> + clear_highpage(folio_page(folio, 0));
> + kvm_gmem_mark_prepared(folio);
> + }
> +
> + vmf->page = folio_file_page(folio, vmf->pgoff);
> +
> +out_folio:
> + if (ret != VM_FAULT_LOCKED) {
> + folio_unlock(folio);
> + folio_put(folio);
> + }
> +
> + return ret;
> +}
> +
> +static const struct vm_operations_struct kvm_gmem_vm_ops = {
> + .fault = kvm_gmem_fault_shared,
> +};
> +
> +static int kvm_gmem_mmap(struct file *file, struct vm_area_struct *vma)
> +{
> + if (!kvm_gmem_supports_shared(file_inode(file)))
> + return -ENODEV;
> +
> + if ((vma->vm_flags & (VM_SHARED | VM_MAYSHARE)) !=
> + (VM_SHARED | VM_MAYSHARE)) {
> + return -EINVAL;
> + }
> +
> + vma->vm_ops = &kvm_gmem_vm_ops;
> +
> + return 0;
> +}
> +
> static struct file_operations kvm_gmem_fops = {
> + .mmap = kvm_gmem_mmap,
> .open = generic_file_open,
> .release = kvm_gmem_release,
> .fallocate = kvm_gmem_fallocate,
> @@ -463,6 +533,9 @@ int kvm_gmem_create(struct kvm *kvm, struct kvm_create_guest_memfd *args)
> u64 flags = args->flags;
> u64 valid_flags = 0;
>
> + if (kvm_arch_supports_gmem_shared_mem(kvm))
> + valid_flags |= GUEST_MEMFD_FLAG_SUPPORT_SHARED;
> +
> if (flags & ~valid_flags)
> return -EINVAL;
>
LGTM!
Reviewed-by: Shivank Garg <shivankg@xxxxxxx>
Thanks,
Shivank
