Re: [RFC v2 PATCH 01/13] mm/shmem: Introduce F_SEAL_GUEST

David Hildenbrand
2021-11-22T15:15:52Z

On 22.11.21 16:09, Jason Gunthorpe wrote:
> On Mon, Nov 22, 2021 at 03:57:17PM +0100, David Hildenbrand wrote:
>> On 22.11.21 15:01, Jason Gunthorpe wrote:
>>> On Mon, Nov 22, 2021 at 02:35:49PM +0100, David Hildenbrand wrote:
>>>> On 22.11.21 14:31, Jason Gunthorpe wrote:
>>>>> On Mon, Nov 22, 2021 at 10:26:12AM +0100, David Hildenbrand wrote:
>>>>>
>>>>>> I do wonder if we want to support sharing such memfds between processes
>>>>>> in all cases ... we most certainly don't want to be able to share
>>>>>> encrypted memory between VMs (I heard that the kernel has to forbid
>>>>>> that). It would make sense in the use case you describe, though.
>>>>>
>>>>> If there is a F_SEAL_XX that blocks every kind of new access, who
>>>>> cares if userspace passes the FD around or not?
>>>> I was imagining that you actually would want to do some kind of "change
>>>> ownership". But yeah, the intended semantics and all use cases we have
>>>> in mind are not fully clear to me yet. If it's really "no new access"
>>>> (side note: is "access" the right word?) then sure, we can pass the fd
>>>> around.
>>>
>>> What is "ownership" in a world with kvm and iommu are reading pages
>>> out of the same fd?
>>
>> In the world of encrypted memory / TDX, KVM somewhat "owns" that memory
>> IMHO (for example, only it can migrate or swap out these pages; it's
>> might be debatable if the TDX module or KVM actually "own" these pages ).
>
> Sounds like it is a swap provider more than an owner?

Yes, I think we can phrase it that way, + "migrate provider"

--
Thanks,

David / dhildenb