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

[Sergio Lopez]

Michael S. Tsirkin <address@hidden> writes:

> 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?

FWIW, for this patch, I'm perfectly fine with changing the "modern"
property to "force-legacy", with "true" as the default value.

>> > 
>> > > 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...

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