Re: [PATCH] virtio: move 'use-disabled-flag' property to hw_compat_4_2

[Stefano Garzarella]

On Mon, Jan 18, 2021 at 04:03:12PM +0000, Dr. David Alan Gilbert wrote:
> * Stefano Garzarella (sgarzare@redhat.com) wrote:
> > Commit 9d7bd0826f introduced a new 'use-disabled-flag' property
> > set to true by default.
> > To allow the migration, we set this property to false in the hw_compat,
> > but in the wrong place (hw_compat_4_1).
> >
> > Since commit 9d7bd0826f was released with QEMU 5.0, we move
> > 'use-disabled-flag' property to hw_compat_4_2, so 4.2 machine types
> > will have the pre-patch behavior and the migration can work.
>
> Be a little careful that fixing this probably causes a migration from
> 5.2->6.0 to fail with this machine type;  so when we do these
> type of fixes we often fix an old machine type between some pair of qemu
> versions and then break it between a different set.

Good point!

I did some tests using the example below always using pc-q35-4.2 and it 
seems that works well:

- 5.2 -> 6.0    pass
- 5.2 -> 4.2            FAIL
- 6.0 -> 5.2    pass
- 6.0 -> 4.2    pass
- 4.2 -> 5.2    pass
- 4.2 -> 6.0    pass

Should I run some more tests?

Thanks,
Stefano

> Dave
>
> > The issue was discovered with vhost-vsock device and 4.2 machine
> > type without running any kernel in the VM:
> >     $ qemu-4.2 -M pc-q35-4.2,accel=kvm \
> >         -device vhost-vsock-pci,guest-cid=4 \
> >         -monitor stdio -incoming tcp:0:3333
> >
> >     $ qemu-5.2 -M pc-q35-4.2,accel=kvm \
> >         -device vhost-vsock-pci,guest-cid=3 \
> >         -monitor stdio
> >     (qemu) migrate -d tcp:0:3333
> >
> >     # qemu-4.2 output
> >     qemu-system-x86_64: Failed to load virtio-vhost_vsock:virtio
> >     qemu-system-x86_64: error while loading state for instance 0x0 of device 
> > '0000:00:03.0/virtio-vhost_vsock'
> >     qemu-system-x86_64: load of migration failed: No such file or directory
> >
> > Reported-by: Jing Zhao <jinzhao@redhat.com>
> > Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=1907255
> > Fixes: 9d7bd0826f ("virtio-pci: disable vring processing when bus-mastering is 
> > disabled")
> > Cc: mdroth@linux.vnet.ibm.com
> > CC: qemu-stable@nongnu.org
> > Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
> > ---
> >  hw/core/machine.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/hw/core/machine.c b/hw/core/machine.c
> > index de3b8f1b31..5d6163ab70 100644
> > --- a/hw/core/machine.c
> > +++ b/hw/core/machine.c
> > @@ -70,12 +70,12 @@ GlobalProperty hw_compat_4_2[] = {
> >      { "qxl", "revision", "4" },
> >      { "qxl-vga", "revision", "4" },
> >      { "fw_cfg", "acpi-mr-restore", "false" },
> > +    { "virtio-device", "use-disabled-flag", "false" },
> >  };
> >  const size_t hw_compat_4_2_len = G_N_ELEMENTS(hw_compat_4_2);
> >
> >  GlobalProperty hw_compat_4_1[] = {
> >      { "virtio-pci", "x-pcie-flr-init", "off" },
> > -    { "virtio-device", "use-disabled-flag", "false" },
> >  };
> >  const size_t hw_compat_4_1_len = G_N_ELEMENTS(hw_compat_4_1);
> >
> > --
> > 2.26.2
> --
> Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK