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

From: Sean Christopherson <[email-masked]>
Date: Thu, 2 Nov 2023 09:03:42 -0700

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, 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.