On 10/2/23 22:38, Alex Williamson wrote: > On Mon, 2 Oct 2023 21:41:55 +0200 > Laszlo Ersek <ler...@redhat.com> wrote: > >> On 10/2/23 21:26, Alex Williamson wrote: >>> On Mon, 2 Oct 2023 20:24:11 +0200 >>> Laszlo Ersek <ler...@redhat.com> wrote: >>> >>>> On 10/2/23 16:41, Alex Williamson wrote: >>>>> On Mon, 2 Oct 2023 15:38:10 +0200 >>>>> Cédric Le Goater <c...@redhat.com> wrote: >>>>> >>>>>> On 10/2/23 13:11, marcandre.lur...@redhat.com wrote: >>>>>>> From: Marc-André Lureau <marcandre.lur...@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.lur...@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...
You didn't misunderstand; I was wrong. I judged this off the compat prop backports to RHEL that I remembered. Your examples from the tree are good evidence. > > 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, ... easy-looking migration patchset becomes quite complex; isn't that the story with almost all QEMU work? :) Thanks! Laszlo