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

[Alex Williamson <alex.williamson@redhat.com>]

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,

Alex

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