On Thu, Nov 2, 2023 at 9:03 AM Sean Christopherson [off-list ref] wrote:

On Thu, Nov 02, 2023, Paolo Bonzini wrote:

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On 10/31/23 23:39, David Matlack wrote:

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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.