Re: [PATCH v3 4/7] memory-attribute-manager: Introduce MemoryAttributeManager to manage RAMBLock with guest_memfd

[Chenyi Qiang <chenyi.qiang@intel.com>]

On 3/18/2025 1:01 AM, Gupta, Pankaj wrote:
> On 3/17/2025 11:36 AM, David Hildenbrand wrote:
>> On 17.03.25 03:54, Chenyi Qiang wrote:
>>>
>>>
>>> On 3/14/2025 8:11 PM, Gupta, Pankaj wrote:
>>>> On 3/10/2025 9:18 AM, Chenyi Qiang wrote:
>>>>> As the commit 852f0048f3 ("RAMBlock: make guest_memfd require
>>>>> uncoordinated discard") highlighted, some subsystems like VFIO may
>>>>> disable ram block discard. However, guest_memfd relies on the discard
>>>>> operation to perform page conversion between private and shared
>>>>> memory.
>>>>> This can lead to stale IOMMU mapping issue when assigning a hardware
>>>>> device to a confidential VM via shared memory. To address this, it is
>>>>> crucial to ensure systems like VFIO refresh its IOMMU mappings.
>>>>>
>>>>> RamDiscardManager is an existing concept (used by virtio-mem) to
>>>>> adjust
>>>>> VFIO mappings in relation to VM page assignment. Effectively page
>>>>> conversion is similar to hot-removing a page in one mode and adding it
>>>>> back in the other. Therefore, similar actions are required for page
>>>>> conversion events. Introduce the RamDiscardManager to guest_memfd to
>>>>> facilitate this process.
>>>>>
>>>>> Since guest_memfd is not an object, it cannot directly implement the
>>>>> RamDiscardManager interface. One potential attempt is to implement
>>>>> it in
>>>>> HostMemoryBackend. This is not appropriate because guest_memfd is per
>>>>> RAMBlock. Some RAMBlocks have a memory backend but others do not. In
>>>>> particular, the ones like virtual BIOS calling
>>>>> memory_region_init_ram_guest_memfd() do not.
>>>>>
>>>>> To manage the RAMBlocks with guest_memfd, define a new object named
>>>>> MemoryAttributeManager to implement the RamDiscardManager
>>>>> interface. The
>>>>
>>>> Isn't this should be the other way around. 'MemoryAttributeManager'
>>>> should be an interface and RamDiscardManager a type of it, an
>>>> implementation?
>>>
>>> We want to use 'MemoryAttributeManager' to represent RAMBlock to
>>> implement the RamDiscardManager interface callbacks because RAMBlock is
>>> not an object. It includes some metadata of guest_memfd like
>>> shared_bitmap at the same time.
>>>
>>> I can't get it that make 'MemoryAttributeManager' an interface and
>>> RamDiscardManager a type of it. Can you elaborate it a little bit? I
>>> think at least we need someone to implement the RamDiscardManager
>>> interface.
>>
>> shared <-> private is translated (abstracted) to "populated <->
>> discarded", which makes sense. The other way around would be wrong.
>>
>> It's going to be interesting once we have more logical states, for
>> example supporting virtio-mem for confidential VMs.
>>
>> Then we'd have "shared+populated, private+populated, shared+discard,
>> private+discarded". Not sure if this could simply be achieved by
>> allowing multiple RamDiscardManager that are effectively chained, or
>> if we'd want a different interface.
> 
> Exactly! In any case generic manager (parent class) would make more
> sense that can work on different operations/states implemented in child
> classes (can be chained as well).

Ah, we are talking about the generic state management. Sorry for my slow
reaction.

So we need to
1. Define a generic manager Interface, e.g.
MemoryStateManager/GenericStateManager.
2. Make RamDiscardManager the child of MemoryStateManager which manages
the state of populated and discarded.
3. Define a new child manager Interface PrivateSharedManager which
manages the state of private and shared.
4. Define a new object ConfidentialMemoryAttribute to implement the
PrivateSharedManager interface.
(Welcome to rename the above Interface/Object)

Is my understanding correct?

> 
> Best regards,
> Pankaj
>>
>