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

[Ackerley Tng <ackerleytng@google.com>]

Ackerley Tng <ackerleytng@google.com> writes:

> Binbin Wu <binbin.wu@linux.intel.com> writes:
>
>> 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@google.com>
>>> Co-developed-by: Vishal Annapurve <vannapurve@google.com>
>>> Signed-off-by: Vishal Annapurve <vannapurve@google.com>
>>> Co-developed-by: Fuad Tabba <tabba@google.com>
>>> Signed-off-by: Fuad Tabba <tabba@google.com>
>>> 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.
>>
>
> Thanks. This should read
>
> If the capability KVM_CAP_GMEM_SHARED_MEM is supported, and
> GUEST_MEMFD_FLAG_SUPPORT_SHARED is specified, then the 'flags' field
> supports GUEST_MEMFD_FLAG_INIT_PRIVATE.
>

My bad, saw your other email. Fixing the above:

If the capability KVM_CAP_GMEM_CONVERSION is supported, and
GUEST_MEMFD_FLAG_SUPPORT_SHARED is specified, then the 'flags' field
supports GUEST_MEMFD_FLAG_INIT_PRIVATE.

>>> 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 .
>>
>>
>
> Yes, thank you! Will also update this to only initialize shareability if
> (flags & GUEST_MEMFD_FLAG_SUPPORT_SHARED).
>
>>> +	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);
>>>   }
>>>   
>>>
>> [...]