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Re: [Qemu-devel] [RFC] virtio-mmio: implement modern (v2) personality (v
From: |
Michael S. Tsirkin |
Subject: |
Re: [Qemu-devel] [RFC] virtio-mmio: implement modern (v2) personality (virtio-1) |
Date: |
Wed, 31 Jul 2019 15:06:11 -0400 |
On Wed, Jul 31, 2019 at 03:55:51PM +0200, Cornelia Huck wrote:
> On Tue, 30 Jul 2019 16:18:52 -0400
> "Michael S. Tsirkin" <address@hidden> wrote:
>
> > 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?
>
> 'force-legacy' is certainly better than 'disable-modern'. Not sure if
> it's much of a gain at this point in time, and it does not really add
> anything over limiting the revision to 0 for ccw, but I don't really
> object to it.
>
> >
> >
> > > >
> > > > > 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.
>
> Most of that stuff is actually in the core code, right? Ripping out
> legacy will have much more impact outside of ccw, I guess.
>
> > Make sure your guests
> > are all up to date in preparation to the day when legacy will go away.
>
> If legacy goes away, legacy guests will be busted anyway :)
It'll take a while for it to go away. But we can try to
push guests in the direction of coding up modern
support e.g. by forcing modern by default.
> (There should not be many, if any, of these -- ccw switched on virtio-1
> by default quite some time ago, and the s390x legacy virtio transport
> was s390-virtio anyway :)
>
> >
> > 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...
- Re: [Qemu-devel] [RFC] virtio-mmio: implement modern (v2) personality (virtio-1), (continued)
- Re: [Qemu-devel] [RFC] virtio-mmio: implement modern (v2) personality (virtio-1), Andrea Bolognani, 2019/07/30
- Re: [Qemu-devel] [RFC] virtio-mmio: implement modern (v2) personality (virtio-1), Cornelia Huck, 2019/07/30
- Re: [Qemu-devel] [RFC] virtio-mmio: implement modern (v2) personality (virtio-1), Andrea Bolognani, 2019/07/30
- Re: [Qemu-devel] [RFC] virtio-mmio: implement modern (v2) personality (virtio-1), Cornelia Huck, 2019/07/30
- Re: [Qemu-devel] [RFC] virtio-mmio: implement modern (v2) personality (virtio-1), Michael S. Tsirkin, 2019/07/30
- Re: [Qemu-devel] [RFC] virtio-mmio: implement modern (v2) personality (virtio-1), Michael S. Tsirkin, 2019/07/30
- Re: [Qemu-devel] [RFC] virtio-mmio: implement modern (v2) personality (virtio-1), Sergio Lopez, 2019/07/31
- Re: [Qemu-devel] [RFC] virtio-mmio: implement modern (v2) personality (virtio-1), Cornelia Huck, 2019/07/31
- Re: [Qemu-devel] [RFC] virtio-mmio: implement modern (v2) personality (virtio-1),
Michael S. Tsirkin <=
Re: [Qemu-devel] [RFC] virtio-mmio: implement modern (v2) personality (virtio-1), Sergio Lopez, 2019/07/31
Re: [Qemu-devel] [RFC] virtio-mmio: implement modern (v2) personality (virtio-1), Laszlo Ersek, 2019/07/30