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

[Hugh Dickins <hughd@google.com>]

On Fri, 19 Aug 2022, Sean Christopherson wrote:
> On Thu, Aug 18, 2022, Kirill A . Shutemov wrote:
> > On Wed, Aug 17, 2022 at 10:40:12PM -0700, Hugh Dickins wrote:
> > > On Wed, 6 Jul 2022, Chao Peng wrote:
> > > 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.
> 
> But QEMU and other VMMs are users of shmem and memfd.  The new features certainly
> aren't useful for _all_ existing users, but I don't think it's fair to say that
> they're not useful for _any_ existing users.

Okay, I stand corrected: there exist some users of memfd_create()
who will also have use for "INACCESSIBLE" memory.

> 
> > > 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 this isn't intended for just TDX (or SNP, or pKVM).  We're not _that_ far off
> from being able to use UPM for "regular" VMs as a way to provide defense-in-depth

UPM? That's an acronym from your side of the fence, I spy references to
it in the mail threads, but haven't tracked down a definition.  I'll
just take it to mean the fd-based memory we're discussing.

> without having to take on the overhead of confidential VMs.  At that point,
> migration and probably even swap are on the table.

Good, the more "flexible" that memory is, the better for competing users
of memory.  But an fd supplied by KVM gives you freedom to change to a
better implementation of allocation underneath, whenever it suits you.
Maybe shmem beneath is good from the start, maybe not.

Hugh