Hi Alex,
> -----Original Message-----
> From: Alex Williamson <alex.williamson@xxxxxxxxxx>
> Sent: Tuesday, January 19, 2021 7:40 AM
> To: Kasireddy, Vivek <vivek.kasireddy@xxxxxxxxx>
> Cc: virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx; Kim, Dongwon <dongwon.kim@xxxxxxxxx>
> 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@xxxxxxxxx> 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);
}
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.
Thanks,
Vivek
> Alex
>
> > Signed-off-by: Vivek Kasireddy <vivek.kasireddy@xxxxxxxxx>
> > ---
> > drivers/vfio/vfio.c | 9 +++++++++
> > 1 file changed, 9 insertions(+)
> >
> > diff --git a/drivers/vfio/vfio.c b/drivers/vfio/vfio.c
> > index 4ad8a35667a7..9fb11b1ad3cd 100644
> > --- a/drivers/vfio/vfio.c
> > +++ b/drivers/vfio/vfio.c
> > @@ -2213,11 +2213,20 @@ static int vfio_unregister_iommu_notifier(struct vfio_group
> *group,
> > return ret;
> > }
> >
> > +extern void vhost_vdmabuf_get_kvm(unsigned long action, void *data);
> > void vfio_group_set_kvm(struct vfio_group *group, struct kvm *kvm)
> > {
> > + void (*fn)(unsigned long, void *);
> > +
> > group->kvm = kvm;
> > blocking_notifier_call_chain(&group->notifier,
> > VFIO_GROUP_NOTIFY_SET_KVM, kvm);
> > +
> > + fn = symbol_get(vhost_vdmabuf_get_kvm);
> > + if (fn) {
> > + fn(VFIO_GROUP_NOTIFY_SET_KVM, kvm);
> > + symbol_put(vhost_vdmabuf_get_kvm);
> > + }
> > }
> > EXPORT_SYMBOL_GPL(vfio_group_set_kvm);
> >
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