Re: [PATCH v7 00/14] KVM: mm: fd-based approach for supporting KVM guest private memory

[Kirill A . Shutemov]

On Wed, Aug 17, 2022 at 10:40:12PM -0700, Hugh Dickins wrote:
> 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.

Migration support is in pipeline. It is part of TDX 1.5 [1]. And swapping
theoretically possible, but I'm not aware of any plans as of now.

[1] https://www.intel.com/content/www/us/en/developer/articles/technical/intel-trust-domain-extensions.html

> Some of these impressions may come from earlier iterations of the
> patchset (v7 looks better in several ways than v5).  I am probably
> underestimating the extent to which you have taken on board other
> usages beyond TDX and SEV private memory, and rightly want to serve
> them all with similar interfaces: perhaps there is enough justification
> for shmem there, but I don't see it.  There was mention of userfaultfd
> in one link: does that provide the justification for using shmem?
> 
> I'm afraid of the special demands you may make of memory allocation
> later on - surprised that huge pages are not mentioned already;
> gigantic contiguous extents? secretmem removed from direct map?

The design allows for extension to hugetlbfs if needed. Combination of
MFD_INACCESSIBLE | MFD_HUGETLB should route this way. There should be zero
implications for shmem. It is going to be separate struct memfile_backing_store.

I'm not sure secretmem is a fit here as we want to extend MFD_INACCESSIBLE
to be movable if platform supports it and secretmem is not migratable by
design (without direct mapping fragmentations).

> Here's what I would prefer, and imagine much easier for you to maintain;
> but I'm no system designer, and may be misunderstanding throughout.
> 
> QEMU gets fd from opening /dev/kvm_something, uses ioctls (or perhaps
> the fallocate syscall interface itself) to allocate and free the memory,
> ioctl for initializing some of it too.  KVM in control of whether that
> fd can be read or written or mmap'ed or whatever, no need to prevent it
> in shmem.c, no need for flags, seals, notifications to and fro because
> KVM is already in control and knows the history.  If shmem actually has
> value, call into it underneath - somewhat like SysV SHM, and /dev/zero
> mmap, and i915/gem make use of it underneath.  If shmem has nothing to
> add, just allocate and free kernel memory directly, recorded in your
> own xarray.

I guess shim layer on top of shmem *can* work. I don't see immediately why
it would not. But I'm not sure it is right direction. We risk creating yet
another parallel VM with own rules/locking/accounting that opaque to
core-mm.

Note that on machines that run TDX guests such memory would likely be the
bulk of memory use. Treating it as a fringe case may bite us one day.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov