RE: [RFC 3/3] vfio: Share the KVM instance with Vdmabuf

["Tian, Kevin" <kevin.tian@intel.com>]

> From: Alex Williamson
> Sent: Wednesday, January 20, 2021 8:51 AM
> 
> On Wed, 20 Jan 2021 00:14:49 +0000
> "Kasireddy, Vivek" <vivek.kasireddy@intel.com> wrote:
> 
> > Hi Alex,
> >
> > > -----Original Message-----
> > > From: Alex Williamson <alex.williamson@redhat.com>
> > > Sent: Tuesday, January 19, 2021 7:40 AM
> > > To: Kasireddy, Vivek <vivek.kasireddy@intel.com>
> > > Cc: virtualization@lists.linux-foundation.org; Kim, Dongwon
> <dongwon.kim@intel.com>
> > > Subject: Re: [RFC 3/3] vfio: Share the KVM instance with Vdmabuf
> > >
> > > On Tue, 19 Jan 2021 00:28:12 -0800
> > > Vivek Kasireddy <vivek.kasireddy@intel.com> wrote:
> > >
> > > > Getting a copy of the KVM instance is necessary for mapping Guest
> > > > pages in the Host.
> > > >
> > > > TODO: Instead of invoking the symbol directly, there needs to be a
> > > > better way of getting a copy of the KVM instance probably by using
> > > > other notifiers. However, currently, KVM shares its instance only
> > > > with VFIO and therefore we are compelled to bind the passthrough'd
> > > > device to vfio-pci.
> > >
> > > Yeah, this is a bad solution, sorry, vfio is not going to gratuitously
> > > call out to vhost to share a kvm pointer.  I'd prefer to get rid of
> > > vfio having any knowledge or visibility of the kvm pointer.  Thanks,
> >
> > [Kasireddy, Vivek] I agree that this is definitely not ideal as I recognize it
> > in the TODO. However, it looks like VFIO also gets a copy of the KVM
> > pointer in a similar manner:
> >
> > virt/kvm/vfio.c
> >
> > static void kvm_vfio_group_set_kvm(struct vfio_group *group, struct kvm
> *kvm)
> > {
> >         void (*fn)(struct vfio_group *, struct kvm *);
> >
> >         fn = symbol_get(vfio_group_set_kvm);
> >         if (!fn)
> >                 return;
> >
> >         fn(group, kvm);
> >
> >         symbol_put(vfio_group_set_kvm);
> > }
> 
> You're equating the mechanism with the architecture.  We use symbols
> here to avoid module dependencies between kvm and vfio, but this is
> just propagating data that userspace is specifically registering
> between kvm and vfio.  vhost doesn't get to piggyback on that channel.
> 
> > With this patch, I am not suggesting that this is a precedent that should be
> followed
> > but it appears there doesn't seem to be an alternative way of getting a copy
> of the KVM
> > pointer that is clean and elegant -- unless I have not looked hard enough. I
> guess we
> > could create a notifier chain with callbacks for VFIO and Vhost that KVM
> would call
> > but this would mean modifying KVM.
> >
> > Also, if I understand correctly, if VFIO does not want to share the KVM
> pointer with
> > VFIO groups, then I think it would break stuff like mdev which counts on it.
> 
> Only kvmgt requires the kvm pointer and the use case there is pretty
> questionable, I wonder if it actually still exists now that we have the
> DMA r/w interface through vfio.  Thanks,
> 

IIRC, kvmgt still needs the kvm pointer to use kvm page tracking interface 
for write-protecting guest pgtable.

Thanks
Kevin
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