Re: [RFC PATCH v2 00/51] 1G page support for guest_memfd

[Sean Christopherson]

On Tue, Jul 08, 2025, Rick P Edgecombe wrote:
> On Tue, 2025-07-08 at 11:03 -0700, Sean Christopherson wrote:
> > > I think there is interest in de-coupling it?
> > 
> > No?
> 
> I'm talking about the intra-host migration/reboot optimization stuff. And not
> doing a good job, sorry.
> 
> >   Even if we get to a point where multiple distinct VMs can bind to a single
> > guest_memfd, e.g. for inter-VM shared memory, there will still need to be a
> > sole
> > owner of the memory.  AFAICT, fully decoupling guest_memfd from a VM would add
> > non-trivial complexity for zero practical benefit.
> 
> I'm talking about moving a gmem fd between different VMs or something using
> KVM_LINK_GUEST_MEMFD [0]. Not advocating to try to support it. But trying to
> feel out where the concepts are headed. It kind of allows gmem fds (or just
> their source memory?) to live beyond a VM lifecycle.

I think the answer is that we want to let guest_memfd live beyond the "struct kvm"
instance, but not beyond the Virtual Machine.  From a past discussion on this topic[*].

 : No go.  Because again, the inode (physical memory) is coupled to the virtual machine
 : as a thing, not to a "struct kvm".  Or more concretely, the inode is coupled to an
 : ASID or an HKID, and there can be multiple "struct kvm" objects associated with a
 : single ASID.  And at some point in the future, I suspect we'll have multiple KVM
 : objects per HKID too.
 : 
 : The current SEV use case is for the migration helper, where two KVM objects share
 : a single ASID (the "real" VM and the helper).  I suspect TDX will end up with
 : similar behavior where helper "VMs" can use the HKID of the "real" VM.  For KVM,
 : that means multiple struct kvm objects being associated with a single HKID.
 : 
 : To prevent use-after-free, KVM "just" needs to ensure the helper instances can't
 : outlive the real instance, i.e. can't use the HKID/ASID after the owning virtual
 : machine has been destroyed.
 : 
 : To put it differently, "struct kvm" is a KVM software construct that _usually_,
 : but not always, is associated 1:1 with a virtual machine.
 : 
 : And FWIW, stashing the pointer without holding a reference would not be a complete
 : solution, because it couldn't guard against KVM reusing a pointer.  E.g. if a
 : struct kvm was unbound and then freed, KVM could reuse the same memory for a new
 : struct kvm, with a different ASID/HKID, and get a false negative on the rebinding
 : check.

Exactly what that will look like in code is TBD, but the concept/logic holds up.

[*] https://lore.kernel.org/all/ZOO782YGRY0YMuPu@xxxxxxxxxx

> [0] https://lore.kernel.org/all/cover.1747368092.git.afranji@xxxxxxxxxx/
> https://lore.kernel.org/kvm/cover.1749672978.git.afranji@xxxxxxxxxx/