On Wed, 6 Jul 2022, Chao Peng wrote:
This is the v7 of this series which tries to implement the fd-based KVM
guest private memory.
Here at last are my reluctant thoughts on this patchset.
fd-based approach for supporting KVM guest private memory: fine.
Use or abuse of memfd and shmem.c: mistaken.
memfd_create() was an excellent way to put together the initial prototype.
But since then, TDX in particular has forced an effort into preventing
(by flags, seals, notifiers) almost everything that makes it shmem/tmpfs.
Are any of the shmem.c mods useful to existing users of shmem.c? No.
Is MFD_INACCESSIBLE useful or comprehensible to memfd_create() users? No.
What use do you have for a filesystem here? Almost none.
IIUC, what you want is an fd through which QEMU can allocate kernel
memory, selectively free that memory, and communicate fd+offset+length
to KVM. And perhaps an interface to initialize a little of that memory
from a template (presumably copied from a real file on disk somewhere).
You don't need shmem.c or a filesystem for that!
If your memory could be swapped, that would be enough of a good reason
to make use of shmem.c: but it cannot be swapped; and although there
are some references in the mailthreads to it perhaps being swappable
in future, I get the impression that will not happen soon if ever.
If your memory could be migrated, that would be some reason to use
filesystem page cache (because page migration happens to understand
that type of memory): but it cannot be migrated.