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Re: [Qemu-devel] No more chameleon devices (was: [PATCH] virtio: Provide


From: Eduardo Habkost
Subject: Re: [Qemu-devel] No more chameleon devices (was: [PATCH] virtio: Provide version-specific variants of virtio PCI devices)
Date: Wed, 17 Oct 2018 16:19:42 -0300
User-agent: Mutt/1.9.2 (2017-12-15)

On Wed, Oct 17, 2018 at 12:38:29PM -0400, Michael S. Tsirkin wrote:
> On Wed, Oct 17, 2018 at 12:56:37PM -0300, Eduardo Habkost wrote:
> > (CCing Marcel, in case he has extra details on the complex
> > Conventional/Express bus/device plugging rules)
> > 
> > On Wed, Oct 17, 2018 at 07:57:39AM +0200, Markus Armbruster wrote:
> > > Laine Stump <address@hidden> writes:
> > > 
> > > [...]
> > > > In the end, having a device that changed PCI ID depending on what kind
> > > > of slot it was plugged into was an idea "too clever for its own good",
> > > > should be avoided when new devices are added in the future, and we
> > > > should at least provide an alternative that doesn't do that for existing
> > > > devices.
> > > 
> > > That means for each chameleon PCI/PCIe device:
> > > 
> > > * create a pair of devices that can only go into one kind of slot
> > > 
> > > * deprecate the chameleon
> > > 
> > > Yes, please!  Volunteers?
> > > 
> > > Do we have similar chameleons outside PCI?
> > 
> > I'm worried that we could be trying to address multiple issues at
> > the same time, and I'm not sure yet if we should address all of
> > them in one take.
> > 
> > Right now we need to differentiate non-transitional and
> > transitional virtio devices, for a few reasons:
> > * They have different PCI IDs;
> > * Legacy drivers don't work with non-transitional devices;
> > * Transitional virtio devices can be plugged to Conventional PCI
> >   buses; non-transitional ones can't.
> 
> No that last point isn't true at all.

Sorry, I got it reversed:

* Non-transitional virtio devices can be plugged to PCI Express
  buses; transitional ones can't[1].

[1] Well, theoretically, under a few circumstances they could.
    We could extend the QMP interfaces in the future to cover
    that use case, but I don't think it's worth the effort.

> 
> > This patch addresses that problem.
> >
> > You seem to be talking about a different issue:
> > * Some devices (including transitional virtio) can be plugged on
> >   both Conventional PCI and PCI Express buses (I will call those
> >   devices "hybrid PCI devices").
> > 
> > The former is a practical problem: management software needs to
> > be able to ask for a transitional virtio device, depending o the
> > guest OS being run.
> > 
> > Addressing the latter seems more complex (it would affect other
> > devices, not just virtio), and I don't see which practical
> > problems it would solve.
> > 
> > I see some problems it wouldn't solve, though: the system
> > wouldn't be able to represent the fact that transitional virtio
> > devices can still work on PCI Express buses, as long as they
> > support PIO bars; or that Conventional PCI devices can be plugged
> > to PCI Express root buses.
> > 
> > I don't see problems caused by hybrid conventional/express PCI
> > devices.  The original problem with virtio devices was just not
> > being hybrid, it was lying about being hybrid: non-transitional
> > virtio devices are hybrid, but transitional virtio devices
> > aren't.
> > 
> > I wouldn't be against abolishing hybrid PCI devices completely if
> > somebody volunteers to do the work.  I just don't see which
> > problems this would solve.
> > 
> > -- 
> > Eduardo

-- 
Eduardo



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