Re: upstream kernel crashes

[Xuan Zhuo]

On Mon, 15 Aug 2022 17:32:06 -0400, "Michael S. Tsirkin" <mst@xxxxxxxxxx> wrote:
> On Mon, Aug 15, 2022 at 01:53:30PM -0700, Andres Freund wrote:
> > Hi,
> >
> > On 2022-08-15 16:21:51 -0400, Michael S. Tsirkin wrote:
> > > On Mon, Aug 15, 2022 at 10:46:17AM -0700, Andres Freund wrote:
> > > > Hi,
> > > >
> > > > On 2022-08-15 12:50:52 -0400, Michael S. Tsirkin wrote:
> > > > > On Mon, Aug 15, 2022 at 09:45:03AM -0700, Andres Freund wrote:
> > > > > > Hi,
> > > > > >
> > > > > > On 2022-08-15 11:40:59 -0400, Michael S. Tsirkin wrote:
> > > > > > > OK so this gives us a quick revert as a solution for now.
> > > > > > > Next, I would appreciate it if you just try this simple hack.
> > > > > > > If it crashes we either have a long standing problem in virtio
> > > > > > > code or more likely a gcp bug where it can't handle smaller
> > > > > > > rings than what device requestes.
> > > > > > > Thanks!
> > > > > >
> > > > > > I applied the below and the problem persists.
> > > > > >
> > > > > > [...]
> > > > >
> > > > > Okay!
> > > >
> > > > Just checking - I applied and tested this atop 6.0-rc1, correct? Or did you
> > > > want me to test it with the 762faee5a267 reverted? I guess what you're trying
> > > > to test if a smaller queue than what's requested you'd want to do so without
> > > > the problematic patch applied...
> > > >
> > > >
> > > > Either way, I did this, and there are no issues that I could observe. No
> > > > oopses, no broken networking. But:
> > > >
> > > > To make sure it does something I added a debugging printk - which doesn't show
> > > > up. I assume this is at a point at least earlyprintk should work (which I see
> > > > getting enabled via serial)?
> > > >
> >
> > > Sorry if I was unclear.  I wanted to know whether the change somehow
> > > exposes a driver bug or a GCP bug. So what I wanted to do is to test
> > > this patch on top of *5.19*, not on top of the revert.
> >
> > Right, the 5.19 part was clear, just the earlier test:
> >
> > > > > > On 2022-08-15 11:40:59 -0400, Michael S. Tsirkin wrote:
> > > > > > > OK so this gives us a quick revert as a solution for now.
> > > > > > > Next, I would appreciate it if you just try this simple hack.
> > > > > > > If it crashes we either have a long standing problem in virtio
> > > > > > > code or more likely a gcp bug where it can't handle smaller
> > > > > > > Thanks!
> >
> > I wasn't sure about.
> >
> > After I didn't see any effect on 5.19 + your patch, I grew a bit suspicious
> > and added the printks.
> >
> >
> > > Yes I think printk should work here.
> >
> > The reason the debug patch didn't change anything, and that my debug printk
> > didn't show, is that gcp uses the legacy paths...
>
> Wait a second. Eureka I think!
>
> So I think GCP is not broken.
> I think what's broken is this patch:
>
> commit cdb44806fca2d0ad29ca644cbf1505433902ee0c
> Author: Xuan Zhuo <xuanzhuo@xxxxxxxxxxxxxxxxx>
> Date:   Mon Aug 1 14:38:54 2022 +0800
>
>     virtio_pci: support the arg sizes of find_vqs()
>
>
> Specifically:
>
> diff --git a/drivers/virtio/virtio_pci_legacy.c b/drivers/virtio/virtio_pci_legacy.c
> index 2257f1b3d8ae..d75e5c4e637f 100644
> --- a/drivers/virtio/virtio_pci_legacy.c
> +++ b/drivers/virtio/virtio_pci_legacy.c
> @@ -112,6 +112,7 @@ static struct virtqueue *setup_vq(struct virtio_pci_device *vp_dev,
>                                   unsigned int index,
>                                   void (*callback)(struct virtqueue *vq),
>                                   const char *name,
> +                                 u32 size,
>                                   bool ctx,
>                                   u16 msix_vec)
>  {
> @@ -125,10 +126,13 @@ static struct virtqueue *setup_vq(struct virtio_pci_device *vp_dev,
>         if (!num || vp_legacy_get_queue_enable(&vp_dev->ldev, index))
>                 return ERR_PTR(-ENOENT);
>
> +       if (!size || size > num)
> +               size = num;
> +
>         info->msix_vector = msix_vec;
>
>         /* create the vring */
> -       vq = vring_create_virtqueue(index, num,
> +       vq = vring_create_virtqueue(index, size,
>                                     VIRTIO_PCI_VRING_ALIGN, &vp_dev->vdev,
>                                     true, false, ctx,
>                                     vp_notify, callback, name);
>
>
>
> So if you pass the size parameter for a legacy device it will
> try to make the ring smaller and that is not legal with
> legacy at all. But the driver treats legacy and modern
> the same, it allocates a smaller queue anyway.
>
>
> Lo and behold, I pass disable-modern=on to qemu and it happily
> corrupts memory exactly the same as GCP does.

Yes, I think you are right.

Thank you very much.

>
>
> So the new find_vqs API is actually completely broken, it can not work for
> legacy at all and for added fun there's no way to find out
> that it's legacy. Maybe we should interpret the patch
>
> So I think I will also revert
>
> 04ca0b0b16f11faf74fa92468dab51b8372586cd..fe3dc04e31aa51f91dc7f741a5f76cc4817eb5b4
>
>
>
>
>
>
>
> > If there were a bug in the legacy path, it'd explain why the problem only
> > shows on gcp, and not in other situations.
> >
> > I'll queue testing the legacy path with the equivalent change.
> >
> > - Andres
> >
> >
> > Greetings,
> >
> > Andres Freund
>