Re: [Qemu-devel] [PATCH for-4.0 v4 2/2] virtio: Provide version-specific variants of virtio PCI devices

[Eduardo Habkost <ehabkost@redhat.com>]

On Thu, Jan 03, 2019 at 11:48:37AM +0100, Cornelia Huck wrote:
> On Thu, 3 Jan 2019 11:14:44 +0100
> Thomas Huth <thuth@redhat.com> wrote:
> 
> > On 2019-01-03 10:38, Thomas Huth wrote:
> > > On 2018-12-05 20:57, Eduardo Habkost wrote:  
> > >> Many of the current virtio-*-pci device types actually represent
> > >> 3 different types of devices:
> > >> * virtio 1.0 non-transitional devices
> > >> * virtio 1.0 transitional devices
> > >> * virtio 0.9 ("legacy device" in virtio 1.0 terminology)
> > >>
> > >> That would be just an annoyance if it didn't break our device/bus
> > >> compatibility QMP interfaces.  With these multi-purpose device
> > >> types, there's no way to tell management software that
> > >> transitional devices and legacy devices require a Conventional
> > >> PCI bus.
> > >>
> > >> The multi-purpose device types would also prevent us from telling
> > >> management software what's the PCI vendor/device ID for them,
> > >> because their PCI IDs change at runtime depending on the bus
> > >> where they were plugged.
> > >>
> > >> This patch adds separate device types for each of those virtio
> > >> device flavors:
> > >>
> > >> - virtio-*-pci: the existing multi-purpose device types
> > >>   - Configurable using `disable-legacy` and `disable-modern`
> > >>     properties
> > >>   - Legacy driver support is automatically enabled/disabled
> > >>     depending on the bus where it is plugged
> > >>   - Supports Conventional PCI and PCI Express buses
> > >>     (but Conventional PCI is incompatible with
> > >>     disable-legacy=off)
> > >>   - Changes PCI vendor/device IDs at runtime
> > >> - virtio-*-pci-transitional: virtio-1.0 device supporting legacy drivers
> > >>   - Supports Conventional PCI buses only, because
> > >>     it has a PIO BAR
> > >> - virtio-*-pci-non-transitional: modern-only
> > >>   - Supports both Conventional PCI and PCI Express buses
> > >>
> > >> The existing TYPE_* macros for these types will point to an
> > >> abstract base type, so existing casts in the code will keep
> > >> working for all variants.
> > >>
> > >> A simple test script (tests/acceptance/virtio_version.py) is
> > >> included, to check if the new device types are equivalent to
> > >> using the `disable-legacy` and `disable-modern` options.
> > >>
> > >> Acked-by: Andrea Bolognani <abologna@redhat.com>
> > >> Reviewed-by: Cornelia Huck <cohuck@redhat.com>
> > >> Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>  
> > > 
> > >  Hi Eduardo,
> > > 
> > > with these new devices, I can trigger an abort on s390x:
> > > 
> > > $ qemu-system-s390x -M s390-ccw-virtio-2.5 -monitor stdio -no-shutdown
> > > QEMU 3.1.50 monitor - type 'help' for more information
> > > (qemu) device_add vhost-scsi-pci-non-transitional
> > > qemu-system-s390x: hw/core/qdev-properties.c:1236:
> > > qdev_prop_set_globals: Assertion `prop->user_provided' failed.
> > > Aborted (core dumped)  
> > FWIW, it happens with x86, too:
> > 
> > $ qemu-system-x86_64 -M pc-i440fx-2.6 -monitor stdio
> > QEMU 3.1.50 monitor - type 'help' for more information
> > (qemu) device_add vhost-scsi-pci-non-transitional
> > qemu-system-x86_64: hw/core/qdev-properties.c:1236:
> > qdev_prop_set_globals: Assertion `prop->user_provided' failed.
> > Aborted (core dumped)
> > 
> > Only machine types newer than 2.7 seem to be OK.
> 
> It also fails for vhost-scsi-pci-transitional on 2.6 and older machines.
> 
> The problem is that HW_COMPAT_2_6 tries to set the disable_modern
> property, which the version-specific variants of virtio-pci do not have.
> 
> Not sure how to fix this. The version-specific devices cannot really
> work on the compat machine.

Oops.

My patch broke the assumption that every virtio-pci subclass is a
generic legacy/modern device.  The good news is that HW_COMPAT_2_6
needs to affect only devices that existed in QEMU
2.6 (not every subclass of virtio-pci).

I see 3 possible ways to address this:

1) Making disable-legacy and disable-modern available on all
virtio-pci subclasses again.

This is simple to implement, but I would like to avoid that.  I'd
prefer to keep the legacy/modern logic complexity restricted to
the old generic virtio devices.


2) Creating a virtio-pci-generic or virtio-pci-hybrid interface
type, add it to generic_type_info at virtio_pci_types_register(),
and use it on HW_COMPAT_2_6 instead of "virtio-pci".

This is simple to implement, but I'm wary.  I'm already bothered
by the complexity of our PCI and virtio type hierarchies.  We
have multiple overlapping sets of virtio-pci devices (depending
which way you look), and specifying exactly which set we want to
affect is tricky.


3) Replacing "virtio-pci" on HW_COMPAT_2_6 with an explicit list
of concrete virtio-pci types that existed when QEMU 2.6 was
released.  This will make HW_COMPAT_2_6 grow, but reduce
complexity of the whole system.  I'm inclined to implement this
solution.

-- 
Eduardo