Re: [PATCH v13 16/35] KVM: Add KVM_CREATE_GUEST_MEMFD ioctl() for guest-specific backing memory

[David Matlack]

On Thu, Nov 2, 2023 at 9:03 AM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
>
> On Thu, Nov 02, 2023, Paolo Bonzini wrote:
> > On 10/31/23 23:39, David Matlack wrote:
> > > > > Maybe can you sketch out how you see this proposal being extensible to
> > > > > using guest_memfd for shared mappings?
> > > > For in-place conversions, e.g. pKVM, no additional guest_memfd is needed.  What's
> > > > missing there is the ability to (safely) mmap() guest_memfd, e.g. KVM needs to
> > > > ensure there are no outstanding references when converting back to private.
> > > >
> > > > For TDX/SNP, assuming we don't find a performant and robust way to do in-place
> > > > conversions, a second fd+offset pair would be needed.
> > > Is there a way to support non-in-place conversions within a single guest_memfd?
> >
> > For TDX/SNP, you could have a hook from KVM_SET_MEMORY_ATTRIBUTES to guest
> > memory.  The hook would invalidate now-private parts if they have a VMA,
> > causing a SIGSEGV/EFAULT if the host touches them.
> >
> > It would forbid mappings from multiple gfns to a single offset of the
> > guest_memfd, because then the shared vs. private attribute would be tied to
> > the offset.  This should not be a problem; for example, in the case of SNP,
> > the RMP already requires a single mapping from host physical address to
> > guest physical address.
>
> I don't see how this can work.  It's not a M:1 scenario (where M is multiple gfns),
> it's a 1:N scenario (wheren N is multiple offsets).  The *gfn* doesn't change on
> a conversion, what needs to change to do non-in-place conversion is the pfn, which
> is effectively the guest_memfd+offset pair.
>
> So yes, we *could* support non-in-place conversions within a single guest_memfd,
> but it would require a second offset,

Why can't KVM free the existing page at guest_memfd+offset and
allocate a new one when doing non-in-place conversions?

> at which point it makes sense to add a
> second file descriptor as well.  Userspace could still use a single guest_memfd
> instance, i.e. pass in the same file descriptor but different offsets.