Re: [RFC PATCH v11 12/29] KVM: Add KVM_CREATE_GUEST_MEMFD ioctl() for guest-specific backing memory

[Fuad Tabba]

Hi Sean,

On Thu, Jul 27, 2023 at 6:13 PM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
>
> On Thu, Jul 27, 2023, Fuad Tabba wrote:
> > Hi Sean,
> >
> > <snip>
> > ...
> >
> > > @@ -5134,6 +5167,16 @@ static long kvm_vm_ioctl(struct file *filp,
> > >         case KVM_GET_STATS_FD:
> > >                 r = kvm_vm_ioctl_get_stats_fd(kvm);
> > >                 break;
> > > +       case KVM_CREATE_GUEST_MEMFD: {
> > > +               struct kvm_create_guest_memfd guest_memfd;
> > > +
> > > +               r = -EFAULT;
> > > +               if (copy_from_user(&guest_memfd, argp, sizeof(guest_memfd)))
> > > +                       goto out;
> > > +
> > > +               r = kvm_gmem_create(kvm, &guest_memfd);
> > > +               break;
> > > +       }
> >
> > I'm thinking line of sight here, by having this as a vm ioctl (rather
> > than a system iocl), would it complicate making it possible in the
> > future to share/donate memory between VMs?
>
> Maybe, but I hope not?
>
> There would still be a primary owner of the memory, i.e. the memory would still
> need to be allocated in the context of a specific VM.  And the primary owner should
> be able to restrict privileges, e.g. allow a different VM to read but not write
> memory.
>
> My current thinking is to (a) tie the lifetime of the backing pages to the inode,
> i.e. allow allocations to outlive the original VM, and (b) create a new file each
> time memory is shared/donated with a different VM (or other entity in the kernel).
>
> That should make it fairly straightforward to provide different permissions, e.g.
> track them per-file, and I think should also avoid the need to change the memslot
> binding logic since each VM would have it's own view/bindings.
>
> Copy+pasting a relevant snippet from a lengthier response in a different thread[*]:
>
>   Conceptually, I think KVM should to bind to the file.  The inode is effectively
>   the raw underlying physical storage, while the file is the VM's view of that
>   storage.

I'm not aware of any implementation of sharing memory between VMs in
KVM before (afaik, since there was no need for one). The following is
me thinking out loud, rather than any strong opinions on my part.

If an allocation can outlive the original VM, then why associate it
with that (or a) VM to begin with? Wouldn't it be more flexible if it
were a system-level construct, which is effectively what it was in
previous iterations of this? This doesn't rule out binding to the
file, and keeping the inode as the underlying physical storage.

The binding of a VM to a guestmem object could happen implicitly with
KVM_SET_USER_MEMORY_REGION2, or we could have a new ioctl specifically
for handling binding.

Cheers,
/fuad


>   Practically, I think that gives us a clean, intuitive way to handle intra-host
>   migration.  Rather than transfer ownership of the file, instantiate a new file
>   for the target VM, using the gmem inode from the source VM, i.e. create a hard
>   link.  That'd probably require new uAPI, but I don't think that will be hugely
>   problematic.  KVM would need to ensure the new VM's guest_memfd can't be mapped
>   until KVM_CAP_VM_MOVE_ENC_CONTEXT_FROM (which would also need to verify the
>   memslots/bindings are identical), but that should be easy enough to enforce.
>
>   That way, a VM, its memslots, and its SPTEs are tied to the file, while allowing
>   the memory and the *contents* of memory to outlive the VM, i.e. be effectively
>   transfered to the new target VM.  And we'll maintain the invariant that each
>   guest_memfd is bound 1:1 with a single VM.
>
>   As above, that should also help us draw the line between mapping memory into a
>   VM (file), and freeing/reclaiming the memory (inode).
>
>   There will be extra complexity/overhead as we'll have to play nice with the
>   possibility of multiple files per inode, e.g. to zap mappings across all files
>   when punching a hole, but the extra complexity is quite small, e.g. we can use
>   address_space.private_list to keep track of the guest_memfd instances associated
>   with the inode.
>
>   Setting aside TDX and SNP for the moment, as it's not clear how they'll support
>   memory that is "private" but shared between multiple VMs, I think per-VM files
>   would work well for sharing gmem between two VMs.  E.g. would allow a give page
>   to be bound to a different gfn for each VM, would allow having different permissions
>   for each file (e.g. to allow fallocate() only from the original owner).
>
> [*] https://lore.kernel.org/all/ZLGiEfJZTyl7M8mS@xxxxxxxxxx
>