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

Alexey Kardashevskiy
2025-01-15T04:06:13Z

On 10/1/25 17:38, Chenyi Qiang wrote:
>
>
> On 1/10/2025 8:58 AM, Alexey Kardashevskiy wrote:
>>
>>
>> On 9/1/25 15:29, Chenyi Qiang wrote:
>>>
>>>
>>> On 1/9/2025 10:55 AM, Alexey Kardashevskiy wrote:
>>>>
>>>>
>>>> On 9/1/25 13:11, Chenyi Qiang wrote:
>>>>>
>>>>>
>>>>> On 1/8/2025 7:20 PM, Alexey Kardashevskiy wrote:
>>>>>>
>>>>>>
>>>>>> On 8/1/25 21:56, Chenyi Qiang wrote:
>>>>>>>
>>>>>>>
>>>>>>> On 1/8/2025 12:48 PM, Alexey Kardashevskiy wrote:
>>>>>>>> On 13/12/24 18:08, Chenyi Qiang wrote:
>>>>>>>>> As the commit 852f0048f3 ("RAMBlock: make guest_memfd require
>>>>>>>>> uncoordinated discard") highlighted, some subsystems like VFIO
>>>>>>>>> might
>>>>>>>>> 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 (unprotected memory
>>>>>>>>> pages). Blocking shared page discard can solve this problem, but it
>>>>>>>>> could cause guests to consume twice the memory with VFIO, which is
>>>>>>>>> not
>>>>>>>>> acceptable in some cases. An alternative solution is to convey
>>>>>>>>> other
>>>>>>>>> systems like VFIO to refresh its outdated IOMMU mappings.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, so the similar work that needs to happen in
>>>>>>>>> response
>>>>>>>>> to virtio-mem changes needs to happen for page conversion events.
>>>>>>>>> Introduce the RamDiscardManager to guest_memfd to achieve it.
>>>>>>>>>
>>>>>>>>> However, guest_memfd is not an object so it cannot directly
>>>>>>>>> implement
>>>>>>>>> the RamDiscardManager interface.
>>>>>>>>>
>>>>>>>>> One solution is to implement the interface in HostMemoryBackend.
>>>>>>>>> Any
>>>>>>>>
>>>>>>>> This sounds about right.
>>
>> btw I am using this for ages:
>>
>> https://github.com/aik/qemu/commit/3663f889883d4aebbeb0e4422f7be5e357e2ee46
>>
>> but I am not sure if this ever saw the light of the day, did not it?
>> (ironically I am using it as a base for encrypted DMA :) )
>
> Yeah, we are doing the same work. I saw a solution from Michael long
> time ago (when there was still
> a dedicated hostmem-memfd-private backend for restrictedmem/gmem)
> (https://github.com/AMDESE/qemu/commit/3bf5255fc48d648724d66410485081ace41d8ee6)
>
> For your patch, it only implement the interface for
> HostMemoryBackendMemfd. Maybe it is more appropriate to implement it for
> the parent object HostMemoryBackend, because besides the
> MEMORY_BACKEND_MEMFD, other backend types like MEMORY_BACKEND_RAM and
> MEMORY_BACKEND_FILE can also be guest_memfd-backed.
>
> Think more about where to implement this interface. It is still
> uncertain to me. As I mentioned in another mail, maybe ram device memory
> region would be backed by guest_memfd if we support TEE IO iommufd MMIO
> in future. Then a specific object is more appropriate. What's your opinion?

I do not know about this. Unlike RAM, MMIO can only do "in-place
conversion" and the interface to do so is not straight forward and VFIO
owns MMIO anyway so the uAPI will be in iommufd, here is a gist of it:

https://github.com/aik/linux/commit/89e45c0404fa5006b2a4de33a4d582adf1ba9831

"guest request" is a communication channel from the VM to the secure FW
(AMD's "PSP") to make MMIO allow encrypted access.

>>
>>>>>>>>
>>>>>>>>> guest_memfd-backed host memory backend can register itself in the
>>>>>>>>> target
>>>>>>>>> MemoryRegion. However, this solution doesn't cover the scenario
>>>>>>>>> where a
>>>>>>>>> guest_memfd MemoryRegion doesn't belong to the HostMemoryBackend,
>>>>>>>>> e.g.
>>>>>>>>> the virtual BIOS MemoryRegion.
>>>>>>>>
>>>>>>>> What is this virtual BIOS MemoryRegion exactly? What does it look
>>>>>>>> like
>>>>>>>> in "info mtree -f"? Do we really want this memory to be DMAable?
>>>>>>>
>>>>>>> virtual BIOS shows in a separate region:
>>>>>>>
>>>>>>>     Root memory region: system
>>>>>>>      0000000000000000-000000007fffffff (prio 0, ram): pc.ram KVM
>>>>>>>      ...
>>>>>>>      00000000ffc00000-00000000ffffffff (prio 0, ram): pc.bios KVM
>>>>>>
>>>>>> Looks like a normal MR which can be backed by guest_memfd.
>>>>>
>>>>> Yes, virtual BIOS memory region is initialized by
>>>>> memory_region_init_ram_guest_memfd() which will be backed by a
>>>>> guest_memfd.
>>>>>
>>>>> The tricky thing is, for Intel TDX (not sure about AMD SEV), the
>>>>> virtual
>>>>> BIOS image will be loaded and then copied to private region.
>>>>> After that,
>>>>> the loaded image will be discarded and this region become useless.
>>>>
>>>> I'd think it is loaded as "struct Rom" and then copied to the MR-
>>>> ram_guest_memfd() which does not leave MR useless - we still see
>>>> "pc.bios" in the list so it is not discarded. What piece of code are you
>>>> referring to exactly?
>>>
>>> Sorry for confusion, maybe it is different between TDX and SEV-SNP for
>>> the vBIOS handling.
>>>
>>> In x86_bios_rom_init(), it initializes a guest_memfd-backed MR and loads
>>> the vBIOS image to the shared part of the guest_memfd MR.
>>> For TDX, it
>>> will copy the image to private region (not the vBIOS guest_memfd MR
>>> private part) and discard the shared part. So, although the memory
>>> region still exists, it seems useless.
>>> It is different for SEV-SNP, correct? Does SEV-SNP manage the vBIOS in
>>> vBIOS guest_memfd private memory?
>>
>> This is what it looks like on my SNP VM (which, I suspect, is the same
>> as yours as hw/i386/pc.c does not distinguish Intel/AMD for this matter):
>
> Yes, the memory region object is created on both TDX and SEV-SNP.
>
>>
>>  Root memory region: system
>>   0000000000000000-00000000000bffff (prio 0, ram): ram1 KVM gmemfd=20
>>   00000000000c0000-00000000000dffff (prio 1, ram): pc.rom KVM gmemfd=27
>>   00000000000e0000-000000001fffffff (prio 0, ram): ram1
>> @00000000000e0000 KVM gmemfd=20
>> ...
>>   00000000ffc00000-00000000ffffffff (prio 0, ram): pc.bios KVM gmemfd=26
>>
>> So the pc.bios MR exists and in use (hence its appearance in "info mtree
>> -f").
>>
>>
>> I added the gmemfd dumping:
>>
>> --- a/system/memory.c
>> +++ b/system/memory.c
>> @@ -3446,6 +3446,9 @@ static void mtree_print_flatview(gpointer key,
>> gpointer value,
>>                  }
>>              }
>>          }
>> +        if (mr->ram_block && mr->ram_block->guest_memfd >= 0) {
>> +            qemu_printf(" gmemfd=%d", mr->ram_block->guest_memfd);
>> +        }
>>
>
> Then I think the virtual BIOS is another case not belonging to
> HostMemoryBackend which convince us to implement the interface in a
> specific object, no?

TBH I have no idea why pc.rom and pc.bios are separate memory regions
but in any case why do these 2 areas need to be treated any different
than the rest of RAM? Thanks,

>>
>>>>
>>>>
>>>>> So I
>>>>> feel like this virtual BIOS should not be backed by guest_memfd?
>>>>
>>>>  From the above it sounds like the opposite, i.e. it should :)
>>>>
>>>>>>
>>>>>>>      0000000100000000-000000017fffffff (prio 0, ram): pc.ram
>>>>>>> @0000000080000000 KVM
>>>>>>
>>>>>> Anyway if there is no guest_memfd backing it and
>>>>>> memory_region_has_ram_discard_manager() returns false, then the MR is
>>>>>> just going to be mapped for VFIO as usual which seems... alright,
>>>>>> right?
>>>>>
>>>>> Correct. As the vBIOS is backed by guest_memfd and we implement the RDM
>>>>> for guest_memfd_manager, the vBIOS MR won't be mapped by VFIO.
>>>>>
>>>>> If we go with the HostMemoryBackend instead of guest_memfd_manager,
>>>>> this
>>>>> MR would be mapped by VFIO. Maybe need to avoid such vBIOS mapping, or
>>>>> just ignore it since the MR is useless (but looks not so good).
>>>>
>>>> Sorry I am missing necessary details here, let's figure out the above.
>>>>
>>>>>
>>>>>>
>>>>>>
>>>>>>> We also consider to implement the interface in HostMemoryBackend, but
>>>>>>> maybe implement with guest_memfd region is more general. We don't
>>>>>>> know
>>>>>>> if any DMAable memory would belong to HostMemoryBackend although at
>>>>>>> present it is.
>>>>>>>
>>>>>>> If it is more appropriate to implement it with HostMemoryBackend,
>>>>>>> I can
>>>>>>> change to this way.
>>>>>>
>>>>>> Seems cleaner imho.
>>>>>
>>>>> I can go this way.
>>>
>>> [...]
>>>
>>>>>>>>> +
>>>>>>>>> +static int guest_memfd_rdm_replay_populated(const
>>>>>>>>> RamDiscardManager
>>>>>>>>> *rdm,
>>>>>>>>> +                                            MemoryRegionSection
>>>>>>>>> *section,
>>>>>>>>> +                                            ReplayRamPopulate
>>>>>>>>> replay_fn,
>>>>>>>>> +                                            void *opaque)
>>>>>>>>> +{
>>>>>>>>> +    GuestMemfdManager *gmm = GUEST_MEMFD_MANAGER(rdm);
>>>>>>>>> +    struct GuestMemfdReplayData data = { .fn =
>>>>>>>>> replay_fn, .opaque =
>>>>>>>>> opaque };
>>>>>>>>> +
>>>>>>>>> +    g_assert(section->mr == gmm->mr);
>>>>>>>>> +    return guest_memfd_for_each_populated_section(gmm, section,
>>>>>>>>> &data,
>>>>>>>>> +
>>>>>>>>> guest_memfd_rdm_replay_populated_cb);
>>>>>>>>> +}
>>>>>>>>> +
>>>>>>>>> +static int guest_memfd_rdm_replay_discarded_cb(MemoryRegionSection
>>>>>>>>> *section, void *arg)
>>>>>>>>> +{
>>>>>>>>> +    struct GuestMemfdReplayData *data = arg;
>>>>>>>>> +    ReplayRamDiscard replay_fn = data->fn;
>>>>>>>>> +
>>>>>>>>> +    replay_fn(section, data->opaque);
>>>>>>>>
>>>>>>>>
>>>>>>>> guest_memfd_rdm_replay_populated_cb() checks for errors though.
>>>>>>>
>>>>>>> It follows current definiton of ReplayRamDiscard() and
>>>>>>> ReplayRamPopulate() where replay_discard() doesn't return errors and
>>>>>>> replay_populate() returns errors.
>>>>>>
>>>>>> A trace would be appropriate imho. Thanks,
>>>>>
>>>>> Sorry, can't catch you. What kind of info to be traced? The errors
>>>>> returned by replay_populate()?
>>>>
>>>> Yeah. imho these are useful as we expect this part to work in general
>>>> too, right? Thanks,
>>>
>>> Something like?
>>>
>>> diff --git a/system/guest-memfd-manager.c b/system/guest-memfd-manager.c
>>> index 6b3e1ee9d6..4440ac9e59 100644
>>> --- a/system/guest-memfd-manager.c
>>> +++ b/system/guest-memfd-manager.c
>>> @@ -185,8 +185,14 @@ static int
>>> guest_memfd_rdm_replay_populated_cb(MemoryRegionSection *section, voi
>>>   {
>>>       struct GuestMemfdReplayData *data = arg;
>>>       ReplayRamPopulate replay_fn = data->fn;
>>> +    int ret;
>>>
>>> -    return replay_fn(section, data->opaque);
>>> +    ret = replay_fn(section, data->opaque);
>>> +    if (ret) {
>>> +        trace_guest_memfd_rdm_replay_populated_cb(ret);
>>> +    }
>>> +
>>> +    return ret;
>>>   }
>>>
>>> How about just adding some error output in
>>> guest_memfd_for_each_populated_section()/
>>> guest_memfd_for_each_discarded_section()
>>> if the cb() (i.e. replay_populate()) returns error?
>>
>> this will do too, yes. Thanks,
>>
>
>
>

--
Alexey