Re: [PATCH 2/7] guest_memfd: Introduce an object to manage the guest-memfd with RamDiscardManager

Alexey Kardashevskiy
2025-01-24T05:57:03Z

On 24/1/25 14:09, Chenyi Qiang wrote:
>
>
> On 1/24/2025 8:15 AM, Alexey Kardashevskiy wrote:
>>
>>
>> On 22/1/25 16:38, Xiaoyao Li wrote:
>>> On 1/22/2025 11:28 AM, Chenyi Qiang wrote:
>>>>
>>>>
>>>> On 1/22/2025 12:35 AM, Peter Xu wrote:
>>>>> On Tue, Jan 21, 2025 at 09:35:26AM +0800, Chenyi Qiang wrote:
>>>>>>
>>>>>>
>>>>>> On 1/21/2025 2:33 AM, Peter Xu wrote:
>>>>>>> On Mon, Jan 20, 2025 at 06:54:14PM +0100, David Hildenbrand wrote:
>>>>>>>> On 20.01.25 18:21, Peter Xu wrote:
>>>>>>>>> On Mon, Jan 20, 2025 at 11:48:39AM +0100, David Hildenbrand wrote:
>>>>>>>>>> Sorry, I was traveling end of last week. I wrote a mail on the
>>>>>>>>>> train and
>>>>>>>>>> apparently it was swallowed somehow ...
>>>>>>>>>>
>>>>>>>>>>>> Not sure that's the right place. Isn't it the (cc) machine
>>>>>>>>>>>> that controls
>>>>>>>>>>>> the state?
>>>>>>>>>>>
>>>>>>>>>>> KVM does, via MemoryRegion->RAMBlock->guest_memfd.
>>>>>>>>>>
>>>>>>>>>> Right; I consider KVM part of the machine.
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>>> It's not really the memory backend, that's just the memory
>>>>>>>>>>>> provider.
>>>>>>>>>>>
>>>>>>>>>>> Sorry but is not "providing memory" the purpose of "memory
>>>>>>>>>>> backend"? :)
>>>>>>>>>>
>>>>>>>>>> Hehe, what I wanted to say is that a memory backend is just
>>>>>>>>>> something to
>>>>>>>>>> create a RAMBlock. There are different ways to create a
>>>>>>>>>> RAMBlock, even
>>>>>>>>>> guest_memfd ones.
>>>>>>>>>>
>>>>>>>>>> guest_memfd is stored per RAMBlock. I assume the state should
>>>>>>>>>> be stored per
>>>>>>>>>> RAMBlock as well, maybe as part of a "guest_memfd state" thing.
>>>>>>>>>>
>>>>>>>>>> Now, the question is, who is the manager?
>>>>>>>>>>
>>>>>>>>>> 1) The machine. KVM requests the machine to perform the
>>>>>>>>>> transition, and the
>>>>>>>>>> machine takes care of updating the guest_memfd state and
>>>>>>>>>> notifying any
>>>>>>>>>> listeners.
>>>>>>>>>>
>>>>>>>>>> 2) The RAMBlock. Then we need some other Object to trigger
>>>>>>>>>> that. Maybe
>>>>>>>>>> RAMBlock would have to become an object, or we allocate
>>>>>>>>>> separate objects.
>>>>>>>>>>
>>>>>>>>>> I'm leaning towards 1), but I might be missing something.
>>>>>>>>>
>>>>>>>>> A pure question: how do we process the bios gmemfds?  I assume
>>>>>>>>> they're
>>>>>>>>> shared when VM starts if QEMU needs to load the bios into it,
>>>>>>>>> but are they
>>>>>>>>> always shared, or can they be converted to private later?
>>>>>>>>
>>>>>>>> You're probably looking for memory_region_init_ram_guest_memfd().
>>>>>>>
>>>>>>> Yes, but I didn't see whether such gmemfd needs conversions
>>>>>>> there.  I saw
>>>>>>> an answer though from Chenyi in another email:
>>>>>>>
>>>>>>> https://lore.kernel.org/all/fc7194ee-ed21-4f6b-
>>>>>>> bf87-147a47f5f074@intel.com/
>>>>>>>
>>>>>>> So I suppose the BIOS region must support private / share
>>>>>>> conversions too,
>>>>>>> just like the rest part.
>>>>>>
>>>>>> Yes, the BIOS region can support conversion as well. I think
>>>>>> guest_memfd
>>>>>> backed memory regions all follow the same sequence during setup time:
>>>>>>
>>>>>> guest_memfd is shared when the guest_memfd fd is created by
>>>>>> kvm_create_guest_memfd() in ram_block_add(), But it will sooner be
>>>>>> converted to private just after kvm_set_user_memory_region() in
>>>>>> kvm_set_phys_mem(). So at the boot time of cc VM, the default
>>>>>> attribute
>>>>>> is private. During runtime, the vBIOS can also do the conversion if it
>>>>>> wants.
>>>>>
>>>>> I see.
>>>>>
>>>>>>
>>>>>>>
>>>>>>> Though in that case, I'm not 100% sure whether that could also be
>>>>>>> done by
>>>>>>> reusing the major guest memfd with some specific offset regions.
>>>>>>
>>>>>> Not sure if I understand you clearly. guest_memfd is per-Ramblock. It
>>>>>> will have its own slot. So the vBIOS can use its own guest_memfd to
>>>>>> get
>>>>>> the specific offset regions.
>>>>>
>>>>> Sorry to be confusing, please feel free to ignore my previous comment.
>>>>> That came from a very limited mindset that maybe one confidential VM
>>>>> should
>>>>> only have one gmemfd..
>>>>>
>>>>> Now I see it looks like it's by design open to multiple gmemfds for
>>>>> each
>>>>> VM, then it's definitely ok that bios has its own.
>>>>>
>>>>> Do you know why the bios needs to be convertable?  I wonder whether
>>>>> the VM
>>>>> can copy it over to a private region and do whatever it wants, e.g.
>>>>> attest
>>>>> the bios being valid.  However this is also more of a pure
>>>>> question.. and
>>>>> it can be offtopic to this series, so feel free to ignore.
>>>>
>>>> AFAIK, the vBIOS won't do conversion after it is set as private at the
>>>> beginning. But in theory, the VM can do the conversion at runtime with
>>>> current implementation. As for why make the vBIOS convertable, I'm also
>>>> uncertain about it. Maybe convenient for managing the private/shared
>>>> status by guest_memfd as it's also converted once at the beginning.
>>>
>>> The reason is just that we are too lazy to implement a variant of
>>> guest memfd for vBIOS that is disallowed to be converted from private
>>> to shared.
>>
>> What is the point in disallowing such conversion in QEMU? On AMD, a
>> malicious HV can try converting at any time and if the guest did not ask
>> for it, it will continue accessing those pages as private and trigger an
>> RMP fault. But if the guest asked for conversion, then it should be no
>> problem to convert to shared. What do I miss about TDX here? Thanks,
>
> Re-read Peter's question, maybe I misunderstood it a little bit.
>
> I thought Peter asked why the vBIOS need to do page conversion since it
> would keep private and no need to convert to shared at runtime. So it is

I suspect there is no need to convert vBIOS but also there is no need to
assume that some memory is never convertable.

> not necessary to manage the vBIOS with guest_memfd-backed memory region
> as it only converts to private once during setup stage. Xiaoyao
> mentioned no need to implement a variant of guest_memfd to convert from
> private to shared. As you said, allowing such conversion won't bring
> security issues.
>
> Now, I assume Peter's real question is, if we can copy the vBIOS to a
> private region and no need to create a specific guest_memfd-backed
> memory region for it?

I guess we can copy it but we have pc.bios and pc.rom in own memory
regions for some reason even for legacy non-secure VMs, for ages, so it
has little or nothing to do with whether vBIOS is in private or shared
memory. Thanks,

>
>>
>>
>>>
>>>>>
>>>>> Thanks,
>>>>>
>>>>
>>>
>>
>

--
Alexey