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

[Paolo Bonzini]

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.

Paolo