Re: [RFC PATCH v2 02/51] KVM: guest_memfd: Introduce and use shareability to guard faulting

[Ira Weiny]

Fuad Tabba wrote:
> Hi,
> 
> On Thu, 3 Jul 2025 at 05:12, Michael Roth <michael.roth@xxxxxxx> wrote:
> >
> > On Wed, Jul 02, 2025 at 05:46:23PM -0700, Vishal Annapurve wrote:
> > > On Wed, Jul 2, 2025 at 4:25 PM Michael Roth <michael.roth@xxxxxxx> wrote:
> > > >
> > > > On Wed, Jun 11, 2025 at 02:51:38PM -0700, Ackerley Tng wrote:
> > > > > Michael Roth <michael.roth@xxxxxxx> writes:
> > > > >
> > > > > > On Wed, May 14, 2025 at 04:41:41PM -0700, Ackerley Tng wrote:

[snip]

> > > > > > The mtree contents seems to get stored in the same manner in either case so
> > > > > > performance-wise only the overhead of a few userspace<->kernel switches
> > > > > > would be saved. Are there any other reasons?
> > > > > >
> > > > > > Otherwise, maybe just settle on SHARED as a documented default (since at
> > > > > > least non-CoCo VMs would be able to reliably benefit) and let
> > > > > > CoCo/GUEST_MEMFD_FLAG_SUPPORT_SHARED VMs set PRIVATE at whatever
> > > > > > granularity makes sense for the architecture/guest configuration.
> > > > > >
> > > > >
> > > > > Because shared pages are split once any memory is allocated, having a
> > > > > way to INIT_PRIVATE could avoid the split and then merge on
> > > > > conversion. I feel that is enough value to have this config flag, what
> > > > > do you think?
> > > > >
> > > > > I guess we could also have userspace be careful not to do any allocation
> > > > > before converting.
> >
> > (Re-visiting this with the assumption that we *don't* intend to use mmap() to
> > populate memory (in which case you can pretty much ignore my previous
> > response))
> >
> > I'm still not sure where the INIT_PRIVATE flag comes into play. For SNP,
> > userspace already defaults to marking everything private pretty close to
> > guest_memfd creation time, so the potential for allocations to occur
> > in-between seems small, but worth confirming.
> >
> > But I know in the past there was a desire to ensure TDX/SNP could
> > support pre-allocating guest_memfd memory (and even pre-faulting via
> > KVM_PRE_FAULT_MEMORY), but I think that could still work right? The
> > fallocate() handling could still avoid the split if the whole hugepage
> > is private, though there is a bit more potential for that fallocate()
> > to happen before userspace does the "manually" shared->private
> > conversion. I'll double-check on that aspect, but otherwise, is there
> > still any other need for it?
> 
> It's not just about performance. I think that the need is more a
> matter of having a consistent API with the hypervisors guest_memfd is
> going to support. Memory in guest_memfd is shared by default, but in
> pKVM for example, it's private by default. Therefore, it would be good
  ^^^^^^^^^^^^^^^^
And Coco's as well right?

Ira

> to have a way to ensure that all guest_memfd allocations can be made
> private from the get-go.
> 
> Cheers,
> /fuad
> 

[snip]