Re: [PATCH v1] s390/virtio_ccw: don't allocate/assign airqs for non-existing queues

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

On Wed, Apr 09, 2025 at 01:12:19PM +0200, David Hildenbrand wrote:
> On 09.04.25 12:56, Michael S. Tsirkin wrote:
> > On Wed, Apr 09, 2025 at 12:46:41PM +0200, David Hildenbrand wrote:
> > > On 07.04.25 23:20, Michael S. Tsirkin wrote:
> > > > On Mon, Apr 07, 2025 at 08:47:05PM +0200, David Hildenbrand wrote:
> > > > > > In my opinion, it makes the most sense to keep the spec as it is and
> > > > > > change QEMU and the kernel to match, but obviously that's not trivial
> > > > > > to do in a way that doesn't break existing devices and drivers.
> > > > > 
> > > > > If only it would be limited to QEMU and Linux ... :)
> > > > > 
> > > > > Out of curiosity, assuming we'd make the spec match the current QEMU/Linux
> > > > > implementation at least for the 3 involved features only, would there be a
> > > > > way to adjust crossvm without any disruption?
> > > > > 
> > > > > I still have the feeling that it will be rather hard to get that all
> > > > > implementations match the spec ... For new features+queues it will be easy
> > > > > to force the usage of fixed virtqueue numbers, but for free-page-hinting and
> > > > > reporting, it's a mess :(
> > > > 
> > > > 
> > > > Still thinking about a way to fix drivers... We can discuss this
> > > > theoretically, maybe?
> > > 
> > > Yes, absolutely. I took the time to do some more digging; regarding drivers
> > > only Linux seems to be problematic.
> > > 
> > > virtio-win, FreeBSD, NetBSD and OpenBSD and don't seem to support
> > > problematic features (free page hinting, free page reporting) in their
> > > virtio-balloon implementations.
> > > 
> > > So from the known drivers, only Linux is applicable.
> > > 
> > > reporting_vq is either at idx 4/3/2
> > > free_page_vq is either at idx 3/2
> > > statsq is at idx2 (only relevant if the feature is offered)
> > > 
> > > So if we could test for the existence of a virtqueue at an idx easily, we
> > > could test from highest-to-smallest idx.
> > > 
> > > But I recall that testing for the existance of a virtqueue on s390x resulted
> > > in the problem/deadlock in the first place ...
> > > 
> > > -- 
> > > Cheers,
> > > 
> > > David / dhildenb
> > 
> > So let's talk about a new feature bit?
> 
> Are you thinking about a new feature that switches between "fixed queue
> indices" and "compressed queue indices", whereby the latter would be the
> legacy default and we would expect all devices to switch to the new
> fixed-queue-indices layout?
> 
> We could make all new features require "fixed-queue-indices".

I see two ways:
1. we make driver behave correctly with in spec and out of spec devices
   and we make qemu behave correctly with in spec and out of spec devices
2. a new feature bit

I prefer 1, and when we add a new feature we can also
document that it should be in spec if negotiated.

My question is if 1 is practical.





> > 
> > Since vqs are probed after feature negotiation, it looks like
> > we could have a feature bit trigger sane behaviour, right?
> 
> In the Linux driver, yes. In QEMU (devices), we add the queues when
> realizing, so we'd need some mechanism to adjust the queue indices based on
> feature negotiation I guess?

Well we can add queues later, nothing prevents that.


> For virtio-balloon it might be doable to simply always create+indicate
> free-page hinting to resolve the issue easily.


OK, so
- for devices, we suggest that basically VIRTIO_BALLOON_F_REPORTING
  only created with VIRTIO_BALLOON_F_FREE_PAGE_HINT and 
  VIRTIO_BALLOON_F_FREE_PAGE_HINT only created with VIRTIO_BALLOON_F_STATS_VQ

I got that.


Now, for drivers.

If the dependency is satisfied as above, no difference.

What should drivers do if not?



I think the thing to do would be to first probe spec compliant
vq numbers? If not there, try with the non compliant version?


However,  you wrote:
> > > But I recall that testing for the existance of a virtqueue on s390x resulted
> > > in the problem/deadlock in the first place ...

I think the deadlock was if trying to *use* a non-existent virtqueue?

This is qemu code:

    case CCW_CMD_READ_VQ_CONF:
        if (check_len) {
            if (ccw.count != sizeof(vq_config)) {
                ret = -EINVAL;
                break;
            }
        } else if (ccw.count < sizeof(vq_config)) {
            /* Can't execute command. */
            ret = -EINVAL;
            break;
        }
        if (!ccw.cda) {
            ret = -EFAULT;
        } else {
            ret = ccw_dstream_read(&sch->cds, vq_config.index);
            if (ret) {
                break;
            }
            vq_config.index = be16_to_cpu(vq_config.index);
            if (vq_config.index >= VIRTIO_QUEUE_MAX) {
                ret = -EINVAL;
                break;
            }
            vq_config.num_max = virtio_queue_get_num(vdev,
                                                     vq_config.index);
            vq_config.num_max = cpu_to_be16(vq_config.num_max);
            ret = ccw_dstream_write(&sch->cds, vq_config.num_max);
            if (!ret) {
                sch->curr_status.scsw.count = ccw.count - sizeof(vq_config);
            }
        }

and

            
int virtio_queue_get_num(VirtIODevice *vdev, int n)
{               
    return vdev->vq[n].vring.num;
}           
            


it seems to happily return vq size with no issues?




> For virtio-fs it might not be that easy.

virtio fs? But it has no features?

> > 
> > I kind of dislike it that we have a feature bit for bugs though.
> > What would be a minimal new feature to add so it does not
> > feel wrong?
> 
> Probably as above: fixed vs. compressed virtqueue indices?
> 
> -- 
> Cheers,
> 
> David / dhildenb