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Re: [Qemu-devel] [PATCH v6 2/7] hw/misc: add vmcoreinfo device
From: |
Eduardo Habkost |
Subject: |
Re: [Qemu-devel] [PATCH v6 2/7] hw/misc: add vmcoreinfo device |
Date: |
Fri, 20 Oct 2017 16:48:20 -0200 |
User-agent: |
Mutt/1.9.0 (2017-09-02) |
On Sun, Oct 15, 2017 at 04:56:28AM +0300, Michael S. Tsirkin wrote:
> On Tue, Oct 10, 2017 at 03:01:10PM -0300, Eduardo Habkost wrote:
> > On Tue, Oct 10, 2017 at 04:06:28PM +0100, Daniel P. Berrange wrote:
> > > On Tue, Oct 10, 2017 at 05:00:18PM +0200, Marc-André Lureau wrote:
> > > > Hi
> > > >
> > > > On Tue, Oct 10, 2017 at 10:31 AM, Daniel P. Berrange
> > > > <address@hidden> wrote:
> > > > > On Tue, Oct 10, 2017 at 12:44:26AM +0300, Michael S. Tsirkin wrote:
> > > > >> On Mon, Oct 09, 2017 at 02:02:18PM +0100, Daniel P. Berrange wrote:
> > > > >> > On Mon, Oct 09, 2017 at 02:43:44PM +0200, Igor Mammedov wrote:
> > > > >> > > On Mon, 9 Oct 2017 12:03:36 +0100
> > > > >> > > "Daniel P. Berrange" <address@hidden> wrote:
> > > > >> > >
> > > > >> > > > On Mon, Sep 11, 2017 at 06:59:24PM +0200, Marc-André Lureau
> > > > >> > > > wrote:
> > > > >> > > > > See docs/specs/vmcoreinfo.txt for details.
> > > > >> > > > >
> > > > >> > > > > "etc/vmcoreinfo" fw_cfg entry is added when using "-device
> > > > >> > > > > vmcoreinfo".
> > > > >> > > >
> > > > >> > > > I'm wondering if you considered just adding the entry to
> > > > >> > > > fw_cfg by
> > > > >> > > > default, without requiring any -device arg ? Unless I'm
> > > > >> > > > misunderstanding,
> > > > >> > > > this doesn't feel like a device to me - its just a well known
> > > > >> > > > bucket
> > > > >> > > > in fw_cfg IIUC ? Obviously its existance would need to be
> > > > >> > > > tied to
> > > > >> > > > the latest machine type for ABI reasons though. The benefit of
> > > > >> > > > this
> > > > >> > > > is that it would "just work" without us having to plumb it
> > > > >> > > > through to
> > > > >> > > > all the downstream applications that use QEMU for mgmt guest
> > > > >> > > > (OpenStack,
> > > > >> > > > oVirt, GNOME Boxes, virt-manager, and countless other mgmt
> > > > >> > > > apps).
> > > > >> > > it follows model set by pvpanic device, it's easier to manage
> > > > >> > > from migration
> > > > >> > > POV, one could use it even for old machine types with new qemu
> > > > >> > > (just by adding
> > > > >> > > device, it makes instance not backwards migratable to old qemu
> > > > >> > > but should work
> > > > >> > > for forward migration) and if user doesn't need it, device could
> > > > >> > > be just omitted
> > > > >> > > from CLI.
> > > > >> >
> > > > >> > Sure but it means that in effect no one will have this
> > > > >> > functionality enabled
> > > > >> > for several years. pvpanic has been around a long time and I
> > > > >> > rarely see it
> > > > >> > present in configured guests :-(
> > > > >> >
> > > > >> >
> > > > >> > Regards,
> > > > >> > Daniel
> > > > >>
> > > > >> libvirt runs with -nodefaults, right? I'd argue pretty strongly
> > > > >> -nodefaults
> > > > >> shouldn't add optional devices anyway.
> > > > >
> > > > > This isn't really adding a device though is it - it is just a well
> > > > > known
> > > > > location in fw_cfg to receive data.
> > > >
> > > > Enabling the device on some configurations by default can be done as a
> > > > follow-up patch. Can we get this series reviewed & merged?
> > >
> > > The problem with the -device approach + turning it on by default is that
> > > there
> > > is no way to turn it off again if you don't want it. eg there's way to
> > > undo
> > > an implicit '-device foo' except via -nodefaults, but since libvirt uses
> > > that
> > > already it would negate the effect of enabling it by default
> > > unconditionally.
> >
> > It's still possible to add a -machine option that can
> > enable/disable automatic creation of the device.
> >
> > But I also don't see why it needs to be implemented using -device
> > if it's not really a device. A boolean machine or fw_cfg
> > property is good enough for that.
>
> It certainly feels like a device. It has state
> (that needs to be migrated), it has a host/guest interface.
(Sorry for the late reply)
That's convincing enough to me. :)
> > >
> > > Your previous approach of "-global fw_cfg.vmcoreinfo=on" is nicer in this
> > > respect, as you can trivially turn it on/off, overriding the default state
> > > in both directions.
> >
> > Both "-global fw_cfg.vmcoreinfo=on|off" and
> > "-machine vmcoreinfo=on|off" sound good enough to me.
>
>
> Certainly not a fw cfg flag. Can be a machine flag I guess
> but then we'd have to open-code each such device.
> And don't forget auto - this is what Daniel asks for.
I'm not sure Daniel is really asking for "auto": he is just
asking for a way to disable the new default. If "vmcoreinfo=off"
and "vmcoreinfo=off" works, there's no need for a user-visible
"auto" value.
(Actually, "auto" values makes compatibility code even messier,
because we would need one additional compat property/field to
tell QEMU what "auto" means on each machine)
--
Eduardo
- Re: [Qemu-devel] [PATCH v6 2/7] hw/misc: add vmcoreinfo device, (continued)
- Re: [Qemu-devel] [PATCH v6 2/7] hw/misc: add vmcoreinfo device, Igor Mammedov, 2017/10/09
- Re: [Qemu-devel] [PATCH v6 2/7] hw/misc: add vmcoreinfo device, Daniel P. Berrange, 2017/10/09
- Re: [Qemu-devel] [PATCH v6 2/7] hw/misc: add vmcoreinfo device, Michael S. Tsirkin, 2017/10/09
- Re: [Qemu-devel] [PATCH v6 2/7] hw/misc: add vmcoreinfo device, Daniel P. Berrange, 2017/10/10
- Re: [Qemu-devel] [PATCH v6 2/7] hw/misc: add vmcoreinfo device, Marc-André Lureau, 2017/10/10
- Re: [Qemu-devel] [PATCH v6 2/7] hw/misc: add vmcoreinfo device, Michael S. Tsirkin, 2017/10/10
- Re: [Qemu-devel] [PATCH v6 2/7] hw/misc: add vmcoreinfo device, Daniel P. Berrange, 2017/10/10
- Re: [Qemu-devel] [PATCH v6 2/7] hw/misc: add vmcoreinfo device, Michael S. Tsirkin, 2017/10/10
- Re: [Qemu-devel] [PATCH v6 2/7] hw/misc: add vmcoreinfo device, Eduardo Habkost, 2017/10/10
- Re: [Qemu-devel] [PATCH v6 2/7] hw/misc: add vmcoreinfo device, Michael S. Tsirkin, 2017/10/14
- Re: [Qemu-devel] [PATCH v6 2/7] hw/misc: add vmcoreinfo device,
Eduardo Habkost <=
- Re: [Qemu-devel] [PATCH v6 2/7] hw/misc: add vmcoreinfo device, Michael S. Tsirkin, 2017/10/14
- Re: [Qemu-devel] [PATCH v6 2/7] hw/misc: add vmcoreinfo device, Eduardo Habkost, 2017/10/10