Re: [RFC PATCH v2 02/51] KVM: guest_memfd: Introduce and use shareability to guard faulting

[Binbin Wu]

On 5/15/2025 7:41 AM, Ackerley Tng wrote:
> Track guest_memfd memory's shareability status within the inode as
> opposed to the file, since it is property of the guest_memfd's memory
> contents.
>
> Shareability is a property of the memory and is indexed using the
> page's index in the inode. Because shareability is the memory's
> property, it is stored within guest_memfd instead of within KVM, like
> in kvm->mem_attr_array.
>
> KVM_MEMORY_ATTRIBUTE_PRIVATE in kvm->mem_attr_array must still be
> retained to allow VMs to only use guest_memfd for private memory and
> some other memory for shared memory.
>
> Not all use cases require guest_memfd() to be shared with the host
> when first created. Add a new flag, GUEST_MEMFD_FLAG_INIT_PRIVATE,
> which when set on KVM_CREATE_GUEST_MEMFD, initializes the memory as
> private to the guest, and therefore not mappable by the
> host. Otherwise, memory is shared until explicitly converted to
> private.
>
> Signed-off-by: Ackerley Tng <ackerleytng@xxxxxxxxxx>
> Co-developed-by: Vishal Annapurve <vannapurve@xxxxxxxxxx>
> Signed-off-by: Vishal Annapurve <vannapurve@xxxxxxxxxx>
> Co-developed-by: Fuad Tabba <tabba@xxxxxxxxxx>
> Signed-off-by: Fuad Tabba <tabba@xxxxxxxxxx>
> Change-Id: If03609cbab3ad1564685c85bdba6dcbb6b240c0f
> ---
>   Documentation/virt/kvm/api.rst |   5 ++
>   include/uapi/linux/kvm.h       |   2 +
>   virt/kvm/guest_memfd.c         | 124 ++++++++++++++++++++++++++++++++-
>   3 files changed, 129 insertions(+), 2 deletions(-)
>
> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> index 86f74ce7f12a..f609337ae1c2 100644
> --- a/Documentation/virt/kvm/api.rst
> +++ b/Documentation/virt/kvm/api.rst
> @@ -6408,6 +6408,11 @@ belonging to the slot via its userspace_addr.
>   The use of GUEST_MEMFD_FLAG_SUPPORT_SHARED will not be allowed for CoCo VMs.
>   This is validated when the guest_memfd instance is bound to the VM.
>   
> +If the capability KVM_CAP_GMEM_CONVERSIONS is supported, then the 'flags' field
> +supports GUEST_MEMFD_FLAG_INIT_PRIVATE.

It seems that the sentence is stale?
Didn't find the definition of KVM_CAP_GMEM_CONVERSIONS.

> Setting GUEST_MEMFD_FLAG_INIT_PRIVATE
> +will initialize the memory for the guest_memfd as guest-only and not faultable
> +by the host.
> +
[...]
>   
>   static int kvm_gmem_init_fs_context(struct fs_context *fc)
> @@ -549,12 +645,26 @@ static const struct inode_operations kvm_gmem_iops = {
>   static struct inode *kvm_gmem_inode_make_secure_inode(const char *name,
>   						      loff_t size, u64 flags)
>   {
> +	struct kvm_gmem_inode_private *private;
>   	struct inode *inode;
> +	int err;
>   
>   	inode = alloc_anon_secure_inode(kvm_gmem_mnt->mnt_sb, name);
>   	if (IS_ERR(inode))
>   		return inode;
>   
> +	err = -ENOMEM;
> +	private = kzalloc(sizeof(*private), GFP_KERNEL);
> +	if (!private)
> +		goto out;
> +
> +	mt_init(&private->shareability);

shareability is defined only when CONFIG_KVM_GMEM_SHARED_MEM enabled, should be done within CONFIG_KVM_GMEM_SHARED_MEM .


> +	inode->i_mapping->i_private_data = private;
> +
> +	err = kvm_gmem_shareability_setup(private, size, flags);
> +	if (err)
> +		goto out;
> +
>   	inode->i_private = (void *)(unsigned long)flags;
>   	inode->i_op = &kvm_gmem_iops;
>   	inode->i_mapping->a_ops = &kvm_gmem_aops;
> @@ -566,6 +676,11 @@ static struct inode *kvm_gmem_inode_make_secure_inode(const char *name,
>   	WARN_ON_ONCE(!mapping_unevictable(inode->i_mapping));
>   
>   	return inode;
> +
> +out:
> +	iput(inode);
> +
> +	return ERR_PTR(err);
>   }
>   
[...]