On Wed, 20 Jan 2021 00:14:49 +0000
"Kasireddy, Vivek" [off-list ref] wrote:

Hi Alex,

quoted

-----Original Message-----
From: Alex Williamson <redacted>
Sent: Tuesday, January 19, 2021 7:40 AM
To: Kasireddy, Vivek <vivek.kasireddy@intel.com>
Cc: virtualization@lists.linux-foundation.org; Kim, Dongwon <redacted>
Subject: Re: [RFC 3/3] vfio: Share the KVM instance with Vdmabuf

On Tue, 19 Jan 2021 00:28:12 -0800
Vivek Kasireddy [off-list ref] wrote:
  

quoted

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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