Re: [PATCH v2 4/5] ramfb: make migration conditional

[Alex Williamson <alex.williamson@redhat.com>]

On Mon, 2 Oct 2023 21:41:55 +0200
Laszlo Ersek <lersek@redhat.com> wrote:

> On 10/2/23 21:26, Alex Williamson wrote:
> > On Mon, 2 Oct 2023 20:24:11 +0200
> > Laszlo Ersek <lersek@redhat.com> wrote:
> >   
> >> On 10/2/23 16:41, Alex Williamson wrote:  
> >>> On Mon, 2 Oct 2023 15:38:10 +0200
> >>> Cédric Le Goater <clg@redhat.com> wrote:
> >>>     
> >>>> On 10/2/23 13:11, marcandre.lureau@redhat.com wrote:    
> >>>>> From: Marc-André Lureau <marcandre.lureau@redhat.com>
> >>>>>
> >>>>> RAMFB migration was unsupported until now, let's make it conditional.
> >>>>> The following patch will prevent machines <= 8.1 to migrate it.
> >>>>>
> >>>>> Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>      
> >>>> Maybe localize the new 'ramfb_migrate' attribute close to 'enable_ramfb'
> >>>> in VFIOPCIDevice. Anyhow,    
> >>>
> >>> Shouldn't this actually be tied to whether the device is migratable
> >>> (which for GVT-g - the only ramfb user afaik - it's not)?  What does it
> >>> mean to have a ramfb-migrate=true property on a device that doesn't
> >>> support migration, or false on a device that does support migration.  I
> >>> don't understand why this is a user controllable property.  Thanks,    
> >>
> >> The comments in <https://bugzilla.redhat.com/show_bug.cgi?id=1859424>
> >> (which are unfortunately not public :/ ) suggest that ramfb migration
> >> was simply forgotten when vGPU migration was implemented. So, "now
> >> that vGPU migration is done", this should be added.
> >>
> >> Comment 8 suggests that the following domain XML snippet
> >>
> >>     <hostdev mode='subsystem' type='mdev' managed='no'
> >> model='vfio-pci' display='on' ramfb='on'> <source>
> >>         <address uuid='b155147a-663a-4009-ae7f-e9a96805b3ce'/>
> >>       </source>
> >>       <alias name='ua-b155147a-663a-4009-ae7f-e9a96805b3ce'/>
> >>       <address type='pci' domain='0x0000' bus='0x07' slot='0x00'  
> >> function='0x0'/> </hostdev>  
> >>
> >> is migratable, but the ramfb device malfunctions on the destination
> >> host.
> >>
> >> There's also a huge QEMU cmdline in comment#0 of the bug; I've not
> >> tried to read that.
> >>
> >> AIUI BTW the property is not for the user to control, it's just a
> >> compat knob for versioned machine types. AIUI those are usually
> >> implemented with such (user-visible / -tweakable) device properties.  
> > 
> > If it's not for user control it's unfortunate that we expose it to the
> > user at all, but should it at least use the "x-" prefix to indicate that
> > it's not intended to be an API?  
> 
> I *think* it was your commit db32d0f43839 ("vfio/pci: Add option to
> disable GeForce quirks", 2018-02-06) that hda introduced me to the "x-"
> prefixed properties!
> 
> For some reason though, machine type compat knobs are never named like
> that, AFAIR.

Maybe I'm misunderstanding your comment, but it appears quite common to
use "x-" prefix things in the compat tables...

GlobalProperty hw_compat_8_0[] = {
    { "migration", "multifd-flush-after-each-section", "on"},
    { TYPE_PCI_DEVICE, "x-pcie-ari-nextfn-1", "on" },
    { TYPE_VIRTIO_NET, "host_uso", "off"},
    { TYPE_VIRTIO_NET, "guest_uso4", "off"},
    { TYPE_VIRTIO_NET, "guest_uso6", "off"},
};
const size_t hw_compat_8_0_len = G_N_ELEMENTS(hw_compat_8_0);

GlobalProperty hw_compat_7_2[] = {
    { "e1000e", "migrate-timadj", "off" },
    { "virtio-mem", "x-early-migration", "false" },
    { "migration", "x-preempt-pre-7-2", "true" },
    { TYPE_PCI_DEVICE, "x-pcie-err-unc-mask", "off" },
};
const size_t hw_compat_7_2_len = G_N_ELEMENTS(hw_compat_7_2);
[etc]

> > It's still odd to think that we can
> > have scenarios of a non-migratable vfio device registering a migratable
> > ramfb, and vice versa, but I suppose in the end it doesn't matter.  
> 
> I do think it matters! For one, if migration is not possible with
> vfio-pci-nohotplug, then how can QE (or anyone else) *test* the patch
> (i.e. that it makes a difference)? In that case, the ramfb_setup() call
> from vfio-pci-nohotplug should just open-code "false" for the
> "migratable" parameter.

Some vfio devices support migration, most don't.  I was thinking
ramfb_setup might be called with something like:

	(vdev->ramfb_migrate && vdev->enable_migration)

so that at least the ramfb migration state matches the device, but I
think ultimately it only saves a little bit of overhead in registering
the vmstate, either one not supporting migration should block migration.

Hmm, since enable_migration is auto/on/off, it seems like device
realize should fail if set to 'on' and ramfb_migrate is false.  I think
that's the only way the device options don't become self contradictory.
Thanks,

Alex