Re: [Qemu-devel] [RFC] virtio-mmio: implement modern (v2) personality (virtio-1)

[Michael S. Tsirkin]

On Tue, Jul 30, 2019 at 03:14:00PM +0200, Cornelia Huck wrote:
> On Tue, 30 Jul 2019 14:17:48 +0200
> Andrea Bolognani <address@hidden> wrote:
> 
> > On Tue, 2019-07-30 at 13:35 +0200, Cornelia Huck wrote:
> > > On Tue, 30 Jul 2019 12:25:30 +0200
> > > Andrea Bolognani <address@hidden> wrote:  
> > > > Can you please make sure virtio-mmio uses the existing interface
> > > > instead of introducing a new one?  
> > > 
> > > FWIW, I really hate virtio-pci's disable-modern/disable-legacy... for a
> > > starter, what is 'modern'? Will we have 'ultra-modern' in the future?  
> > 
> > AIUI the modern/legacy terminology is part of the VirtIO spec, so
> > while I agree that it's not necessarily the least prone to ambiguity
> > at least it's well defined.
> 
> Legacy is, modern isn't :) Devices/drivers are conforming to the
> standard, I don't think there's a special term for that.

Right, if we followed the spec, disable-modern would have been
force-legacy.

I'm fine with adding force-legacy for everyone and asking tools to
transition if there. Document it's same as disable-modern for pci.
Cornelia?


> > 
> > > It is also quite backwards with the 'disable' terminology.  
> > 
> > That's also true. I never claimed the way virtio-pci does it is
> > perfect!
> > 
> > > We also have a different mechanism for virtio-ccw ('max_revision',
> > > which covers a bit more than virtio-1; it doesn't have a 'min_revision',
> > > as negotiating the revision down is fine), so I don't see why
> > > virtio-mmio should replicate the virtio-pci mechanism.
> > > 
> > > Also, IIUC, virtio-mmio does not have transitional devices, but either
> > > version 1 (legacy) or version 2 (virtio-1). It probably makes more
> > > sense to expose the device version instead; either as an exact version
> > > (especially if it isn't supposed to go up without incompatible
> > > changes), or with some min/max concept (where version 1 would stand a
> > > bit alone, so that would probably be a bit awkward.)  
> > 
> > I think that if reinventing the wheel is generally agreed not to be
> > a good idea, then it stands to reason that reinventing it twice can
> > only be described as absolute madness :)
> > 
> > We should have a single way to control the VirtIO protocol version
> > that works for all VirtIO devices, regardless of transport. We might
> > even want to have virtio-*-{device,ccw}-non-transitional to mirror
> > the existing virtio-*-pci-non-transitional.
> > 
> > FWIW, libvirt already implements support for (non)-transitional
> > virtio-pci devices using either the dedicated devices or the base
> > virtio-pci plus the disable-{modern,legacy} attributes.
> 
> One problem (besides my dislike of the existing virtio-pci
> interfaces :) is that pci, ccw, and mmio all have slightly different
> semantics.
> 
> - pci: If we need to keep legacy support around, we cannot enable some
>   features (IIRC, pci-e, maybe others as well.) That means transitional
>   devices are in some ways inferior to virtio-1 only devices, so it
>   makes a lot of sense to be able to configure devices without legacy
>   support. The differences between legacy and virtio-1 are quite large.
> - ccw: Has revisions negotiated between device and driver; virtio-1
>   requires revision 1 or higher. (Legacy drivers that don't know the
>   concept of revisions automatically get revision 0.) Differences
>   between legacy and virtio-1 are mostly virtqueue endianness and some
>   control structures.
> - mmio: Has device versions offered by the device, the driver can take
>   it or leave it. No transitional devices. Differences don't look as
>   large as the ones for pci, either.
> 
> So, if we were to duplicate the same scheme as for pci for ccw and mmio
> as well, we'd get
> 
> - ccw: devices that support revision 0 only (disable-modern), that act
>   as today, or that support at least revision 1 (disable-legacy). We
>   still need to keep max_revision around for backwards compatibility.
>   Legacy only makes sense for compat machines (although this is
>   equivalent to max_revision 0); I don't see a reason why you would
>   want virtio-1 only devices, unless you'd want to rip out legacy
>   support in QEMU completely.

Reduce security attack surface slightly. Save some cycles
(down the road) on branches in the endian-ness handling.
Make sure your guests
are all up to date in preparation to the day when legacy will go away.

Not a huge win, for sure, but hey - it's something.

> - mmio: devices that support version 1 (disable-modern), or version 2
>   (disable-legacy). You cannot have both at the same time. Whether this
>   makes sense depends on whether there will be a version 3 in the
>   future.
> 
> So, this might make some sense for mmio; for ccw, I don't see any
> advantages other than confusing people further...