Re: [PATCH vhost v9 2/6] virtio: remove support for names array entries being null.

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Jun 20, 2024 at 04:39:38PM +0800, Xuan Zhuo wrote:
> On Thu, 20 Jun 2024 04:02:45 -0400, "Michael S. Tsirkin" <mst@redhat.com> wrote:
> > On Wed, Apr 24, 2024 at 05:15:29PM +0800, Xuan Zhuo wrote:
> > > commit 6457f126c888 ("virtio: support reserved vqs") introduced this
> > > support. Multiqueue virtio-net use 2N as ctrl vq finally, so the logic
> > > doesn't apply. And not one uses this.
> > >
> > > On the other side, that makes some trouble for us to refactor the
> > > find_vqs() params.
> > >
> > > So I remove this support.
> > >
> > > Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
> > > Acked-by: Jason Wang <jasowang@redhat.com>
> > > Acked-by: Eric Farman <farman@linux.ibm.com> # s390
> > > Acked-by: Halil Pasic <pasic@linux.ibm.com>
> >
> >
> > I don't mind, but this patchset is too big already.
> > Why do we need to make this part of this patchset?
> 
> 
> If some the pointers of the names is NULL, then in the virtio ring,
> we will have a trouble to index from the arrays(names, callbacks...).
> Becasue that the idx of the vq is not the index of these arrays.
> 
> If the names is [NULL, "rx", "tx"], the first vq is the "rx", but index of the
> vq is zero, but the index of the info of this vq inside the arrays is 1.


Ah. So actually, it used to work.

What this should refer to is

commit ddbeac07a39a81d82331a312d0578fab94fccbf1
Author: Wei Wang <wei.w.wang@intel.com>
Date:   Fri Dec 28 10:26:25 2018 +0800

    virtio_pci: use queue idx instead of array idx to set up the vq
    
    When find_vqs, there will be no vq[i] allocation if its corresponding
    names[i] is NULL. For example, the caller may pass in names[i] (i=4)
    with names[2] being NULL because the related feature bit is turned off,
    so technically there are 3 queues on the device, and name[4] should
    correspond to the 3rd queue on the device.
    
    So we use queue_idx as the queue index, which is increased only when the
    queue exists.
    
    Signed-off-by: Wei Wang <wei.w.wang@intel.com>
    Signed-off-by: Michael S. Tsirkin <mst@redhat.com>


Which made it so setting names NULL actually does not reserve a vq.

But I worry about non pci transports - there's a chance they used
a different index with the balloon. Did you test some of these?

-- 
MST