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Message-ID: <diqz34cn6tll.fsf@ackerleytng-ctop.c.googlers.com>
Date: Thu, 29 May 2025 13:37:58 -0700
From: Ackerley Tng <ackerleytng@...gle.com>
To: Binbin Wu <binbin.wu@...ux.intel.com>
Cc: kvm@...r.kernel.org, linux-mm@...ck.org, linux-kernel@...r.kernel.org,
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Subject: Re: [RFC PATCH v2 02/51] KVM: guest_memfd: Introduce and use
shareability to guard faulting
Ackerley Tng <ackerleytng@...gle.com> writes:
> Binbin Wu <binbin.wu@...ux.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@...gle.com>
>>> Co-developed-by: Vishal Annapurve <vannapurve@...gle.com>
>>> Signed-off-by: Vishal Annapurve <vannapurve@...gle.com>
>>> Co-developed-by: Fuad Tabba <tabba@...gle.com>
>>> Signed-off-by: Fuad Tabba <tabba@...gle.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);
>>> }
>>>
>>>
>> [...]
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