On Thu, Jun 02, 2022, Chao Peng wrote: > On Wed, Jun 01, 2022 at 02:11:42PM +0200, Gupta, Pankaj wrote: > > > > > > > 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&data=05%7C01%7Cpankaj.gupta%40amd.com%7Cb81ef334e2dd44c6143308da43b87d17%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637896756895977587%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000%7C%7C%7C&sdata=oQbM2Hj7GlhJTwnTM%2FPnwsfJlmTL7JR9ULBysAqm6V8%3D&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? > > TDX code is here: > https://patchwork.kernel.org/project/kvm/patch/70ed041fd47c1f7571aa259450b3f9244edda48d.1651774250.git.isaku.yamahata@xxxxxxxxx/ > > AFAICS It might be possible to filter that out to a common function. But > would like to hear from Paolo/Sean for their opinion. Eh, I wouldn't put too much effort into creating a common helper, I would be very surprised if TDX and SNP can share a meaningful amount of code that isn't already shared, e.g. provided by MMU helpers. The only part I truly care about sharing is whatever ioctl(s) get added, i.e. I don't want to end up with two ioctls that do the same thing for TDX vs. SNP.
