Re: [RFC] KVM: mm: fd-based approach for supporting KVM guest private memory

[Chao Peng]

On Wed, Sep 15, 2021 at 05:11:47PM +0300, Kirill A. Shutemov wrote:
> On Wed, Sep 15, 2021 at 07:58:57PM +0000, Chao Peng wrote:
> > On Fri, Sep 10, 2021 at 08:18:11PM +0300, Kirill A. Shutemov wrote:
> > > On Fri, Sep 03, 2021 at 12:15:51PM -0700, Andy Lutomirski wrote:
> > > > On 9/3/21 12:14 PM, Kirill A. Shutemov wrote:
> > > > > On Thu, Sep 02, 2021 at 08:33:31PM +0000, Sean Christopherson wrote:
> > > > >> Would requiring the size to be '0' at F_SEAL_GUEST time solve that problem?
> > > > > 
> > > > > I guess. Maybe we would need a WRITE_ONCE() on set. I donno. I will look
> > > > > closer into locking next.
> > > > 
> > > > We can decisively eliminate this sort of failure by making the switch
> > > > happen at open time instead of after.  For a memfd-like API, this would
> > > > be straightforward.  For a filesystem, it would take a bit more thought.
> > > 
> > > I think it should work fine as long as we check seals after i_size in the
> > > read path. See the comment in shmem_file_read_iter().
> > > 
> > > Below is updated version. I think it should be good enough to start
> > > integrate with KVM.
> > > 
> > > I also attach a test-case that consists of kernel patch and userspace
> > > program. It demonstrates how it can be integrated into KVM code.
> > > 
> > > One caveat I noticed is that guest_ops::invalidate_page_range() can be
> > > called after the owner (struct kvm) has being freed. It happens because
> > > memfd can outlive KVM. So the callback has to check if such owner exists,
> > > than check that there's a memslot with such inode.
> > 
> > Would introducing memfd_unregister_guest() fix this?
> 
> I considered this, but it get complex quickly.
> 
> At what point it gets called? On KVM memslot destroy?

I meant when the VM gets destroyed.

> 
> What if multiple KVM slot share the same memfd? Add refcount into memfd on
> how many times the owner registered the memfd?
> 
> It would leave us in strange state: memfd refcount owners (struct KVM) and
> KVM memslot pins the struct file. Weird refcount exchnage program.
> 
> I hate it.

But yes agree things will get much complex in practice.

> 
> > > I guess it should be okay: we have vm_list we can check owner against.
> > > We may consider replace vm_list with something more scalable if number of
> > > VMs will get too high.
> > > 
> > > Any comments?
> > > 
> > > diff --git a/include/linux/memfd.h b/include/linux/memfd.h
> > > index 4f1600413f91..3005e233140a 100644
> > > --- a/include/linux/memfd.h
> > > +++ b/include/linux/memfd.h
> > > @@ -4,13 +4,34 @@
> > >  
> > >  #include <linux/file.h>
> > >  
> > > +struct guest_ops {
> > > +	void (*invalidate_page_range)(struct inode *inode, void *owner,
> > > +				      pgoff_t start, pgoff_t end);
> > > +};
> > 
> > I can see there are two scenarios to invalidate page(s), when punching a
> > hole or ftruncating to 0, in either cases KVM should already been called
> > with necessary information from usersapce with memory slot punch hole
> > syscall or memory slot delete syscall, so wondering this callback is
> > really needed.
> 
> So what you propose? Forbid truncate/punch from userspace and make KVM
> handle punch hole/truncate from within kernel? I think it's layering
> violation.

As far as I understand the flow for punch hole/truncate in this design,
there will be two steps for userspace:
  1. punch hole/delete kvm memory slot, and then
  2. puncn hole/truncate on the memory backing store fd.

In concept we can do whatever needed for invalidation in either steps.
If we can do the invalidation in step 1 then we don’t need bothering
this callback. This is what I mean but agree the current callback can
also work.

> 
> > > +
> > > +struct guest_mem_ops {
> > > +	unsigned long (*get_lock_pfn)(struct inode *inode, pgoff_t offset);
> > > +	void (*put_unlock_pfn)(unsigned long pfn);
> > 
> > Same as above, I’m not clear on which time put_unlock_pfn() would be
> > called, I’m thinking the page can be put_and_unlock when userspace
> > punching a hole or ftruncating to 0 on the fd.
> 
> No. put_unlock_pfn() has to be called after the pfn is in SEPT. This way
> we close race between SEPT population and truncate/punch. get_lock_pfn()
> would stop truncate untile put_unlock_pfn() called.

Okay, makes sense.

Thanks,
Chao