[Qemu-devel] Re: [PATCHv2 09/12] vhost: vhost net support

[Anthony Liguori]

On 02/26/2010 08:49 AM, Michael S. Tsirkin wrote:
> KVM code needs all kind of work-arounds for KVM specific issues.
> It also assumes that KVM is registered at startup, so it
> does not try to optimize finding slots.
>    

No, the slot mapping changes dynamically so KVM certainly needs to 
optimize this.

But the point is, why can't we keep a central list of slots somewhere 
that KVM and vhost-net can both use?  I'm not saying we use a common 
function to do this work, I'm saying qemu should maintain a proper slot 
list than anyone can access.

> I propose merging this as is, and then someone who has an idea
> how to do this better can come and unify the code.
>    

Like I said, this has been a huge source of very subtle bugs in the 
past.  I'm open to hearing what other people think, but I'm concerned 
that if we merge this code, we'll end up facing some nasty bugs that 
could easily be eliminated by just using the code in kvm-all that has 
already been tested rather extensively.

There really aren't that many work-arounds in the code BTW.  The work 
arounds just result in a couple of extra slots too so they shouldn't be 
a burden to vhost.

> Mine has no bugs, let's switch to it!
>
> Seriously, need to tread very carefully here.
> This is why I say: merge it, then look at how to reuse code.
>    

Once it's merged, there's no incentive to look at reusing code.  Again, 
I don't think this is a huge burden to vhost.  The two bits of code 
literally do exactly the same thing.  They just use different data 
structures that ultimately contain the same values.

> > C++ habits die hard :-)
> >      
>
> What's that about?
>    

'++i' is an odd thing to do in C in a for() loop.  We're not explicit 
about it in Coding Style but the vast majority of code just does 'i++'.

> > > +    vq->desc = cpu_physical_memory_map(a,&l, 0);
> > > +    if (!vq->desc || l != s) {
> > > +        r = -ENOMEM;
> > > +        goto fail_alloc;
> > > +    }
> > > +    s = l = offsetof(struct vring_avail, ring) +
> > > +        sizeof(u_int64_t) * vq->num;
> > > +    a = virtio_queue_get_avail(vdev, idx);
> > > +    vq->avail = cpu_physical_memory_map(a,&l, 0);
> > > +    if (!vq->avail || l != s) {
> > > +        r = -ENOMEM;
> > > +        goto fail_alloc;
> > > +    }
> > >
> > >        
> > You don't unmap avail/desc on failure.  map() may fail because the ring
> > cross MMIO memory and you run out of a bounce buffer.
> >
> > IMHO, it would be better to attempt to map the full ring at once and
> > then if that doesn't succeed, bail out.  You can still pass individual
> > pointers via vhost ioctls but within qemu, it's much easier to deal with
> > the whole ring at a time.
> >      
> + a = virtio_queue_get_desc(vdev, idx);
> I prefer to keep as much logic about ring layout as possible
> in virtio.c
>    

Well, the downside is that you need to deal with the error path and 
cleanup paths and it becomes more complicated.

> > > +    s = l = offsetof(struct vring_used, ring) +
> > > +        sizeof(struct vring_used_elem) * vq->num;
> > >
> > >        
> > This is unfortunate.  We redefine this structures in qemu to avoid
> > depending on Linux headers.
> >      
> And we should for e.g. windows portability.
>
>    
> >   But you're using the linux versions instead
> > of the qemu versions.  Is it really necessary for vhost.h to include
> > virtio.h?
> >      
> Yes. And anyway, vhost does not exist on non-linux systems so there
> is no issue IMO.
>    

Yeah, like I said, it's unfortunate because it means a read of vhost and 
a reader of virtio.c is likely to get confused.  I'm not saying there's 
an easy solution, it's just unfortunate.

> > > +    vq->used_phys = a = virtio_queue_get_used(vdev, idx);
> > > +    vq->used = cpu_physical_memory_map(a,&l, 1);
> > > +    if (!vq->used || l != s) {
> > > +        r = -ENOMEM;
> > > +        goto fail_alloc;
> > > +    }
> > > +
> > > +    r = vhost_virtqueue_set_addr(dev, vq, idx, dev->log_enabled);
> > > +    if (r<   0) {
> > > +        r = -errno;
> > > +        goto fail_alloc;
> > > +    }
> > > +    if (!vdev->binding->guest_notifier || !vdev->binding->host_notifier) {
> > > +        fprintf(stderr, "binding does not support irqfd/queuefd\n");
> > > +        r = -ENOSYS;
> > > +        goto fail_alloc;
> > > +    }
> > > +    r = vdev->binding->guest_notifier(vdev->binding_opaque, idx, true);
> > > +    if (r<   0) {
> > > +        fprintf(stderr, "Error binding guest notifier: %d\n", -r);
> > > +        goto fail_guest_notifier;
> > > +    }
> > > +
> > > +    r = vdev->binding->host_notifier(vdev->binding_opaque, idx, true);
> > > +    if (r<   0) {
> > > +        fprintf(stderr, "Error binding host notifier: %d\n", -r);
> > > +        goto fail_host_notifier;
> > > +    }
> > > +
> > > +    file.fd = event_notifier_get_fd(virtio_queue_host_notifier(q));
> > > +    r = ioctl(dev->control, VHOST_SET_VRING_KICK,&file);
> > > +    if (r) {
> > > +        goto fail_kick;
> > > +    }
> > > +
> > > +    file.fd = event_notifier_get_fd(virtio_queue_guest_notifier(q));
> > > +    r = ioctl(dev->control, VHOST_SET_VRING_CALL,&file);
> > > +    if (r) {
> > > +        goto fail_call;
> > > +    }
> > >
> > >        
> > This function would be a bit more reasonable if it were split into
> > sections FWIW.
> >      
> Not sure what do you mean here.
>    

Just a suggestion.  For instance, moving the setting up of the notifiers 
to a separate function would help with readability IMHO.

> > You never unmap() the mapped memory and you're cheating by assuming that
> > the virtio rings have a constant mapping for the life time of a guest.
> > That's not technically true.  My concern is that since a guest can
> > trigger remappings (by adjusting PCI mappings) badness can ensue.
> >      
> I do not know how this can happen. What do PCI mappings have to do with this?
> Please explain. If it can, vhost will need notification to update.
>    

If a guest modifies the bar for an MMIO region such that it happens to 
exist in RAM, while this is a bad thing for the guest to do, I don't 
think we do anything to stop it.  When the region gets remapped, the 
result will be that the mapping will change.

Within qemu, because we carry the qemu_mutex, we know that the mappings 
are fixed as long as we're in qemu.  We're very careful to assume that 
we don't rely on a mapping past when we drop the qemu_mutex.

With vhost, you register a slot table and update it whenever mappings 
change.  I think that's good enough for dealing with ram addresses.  But 
you pass the virtual address for the rings and assume those mappings 
never change.

I'm pretty sure a guest can cause those to change and I'm not 100% sure, 
but I think it's a potential source of exploits if you assume a 
mapping.  In the very least, a guest can trick vhost into writing to ram 
that it wouldn't normally write to.

> > If you're going this way, I'd suggest making static inlines in the
> > header file instead of polluting the C file.  It's more common to search
> > within a C file and having two declarations can get annoying.
> >
> > Regards,
> >
> > Anthony Liguori
> >      
> The issue with inline is that this means that virtio net will depend on
> target (need to be recompiled).  As it is, a single object can link with
> vhost and non-vhost versions.
>    

Fair enough.

Regards,

Anthony Liguori