Re: [PATCH v6 3/8] mm/memfd: Introduce MFD_INACCESSIBLE flag

Gupta, Pankaj
2022-06-01T12:12:15Z

>>> Introduce a new memfd_create() flag indicating the content of the
>>> created memfd is inaccessible from userspace through ordinary MMU
>>> access (e.g., read/write/mmap). However, the file content can be
>>> accessed via a different mechanism (e.g. KVM MMU) indirectly.
>>>
>>
>> SEV, TDX, pkvm and software-only VMs seem to have usecases to set up
>> initial guest boot memory with the needed blobs.
>> TDX already supports a KVM IOCTL to transfer contents to private
>> memory using the TDX module but rest of the implementations will need
>> to invent
>> a way to do this.
>
> There are some discussions in https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Flkml.org%2Flkml%2F2022%2F5%2F9%2F1292&amp;data=05%7C01%7Cpankaj.gupta%40amd.com%7Cb81ef334e2dd44c6143308da43b87d17%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637896756895977587%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000%7C%7C%7C&amp;sdata=oQbM2Hj7GlhJTwnTM%2FPnwsfJlmTL7JR9ULBysAqm6V8%3D&amp;reserved=0
> already. I somehow agree with Sean. TDX is using an dedicated ioctl to
> copy guest boot memory to private fd so the rest can do that similarly.
> The concern is the performance (extra memcpy) but it's trivial since the
> initial guest payload is usually optimized in size.
>
>>
>> Is there a plan to support a common implementation for either allowing
>> initial write access from userspace to private fd or adding a KVM
>> IOCTL to transfer contents to such a file,
>> as part of this series through future revisions?
>
> Indeed, adding pre-boot private memory populating on current design
> isn't impossible, but there are still some opens, e.g. how to expose
> private fd to userspace for access, pKVM and CC usages may have
> different requirements. Before that's well-studied I would tend to not
> add that and instead use an ioctl to copy. Whether we need a generic
> ioctl or feature-specific ioctl, I don't have strong opinion here.
> Current TDX uses a feature-specific ioctl so it's not covered in this
> series.

Common function or ioctl to populate preboot private memory actually
makes sense.

Sorry, did not follow much of TDX code yet, Is it possible to filter out
the current TDX specific ioctl to common function so that it can be used
by other technologies?

Thanks,
Pankaj