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.
