Paul Durrant
2013-Jun-19 14:00 UTC
Re: [Qemu-devel] [PATCH] Remove hardcoded xen-platform device initialization
> -----Original Message----- > From: qemu-devel-bounces+paul.durrant=citrix.com@nongnu.org > [mailto:qemu-devel-bounces+paul.durrant=citrix.com@nongnu.org] On > Behalf Of Stefano Stabellini > Sent: 19 June 2013 14:53 > To: Ian Campbell > Cc: Paolo Bonzini; Paul Durrant; xen-devel@lists.xen.org; qemu- > devel@nongnu.org; Stefano Stabellini > Subject: Re: [Qemu-devel] [Xen-devel] [PATCH] Remove hardcoded xen- > platform device initialization > > On Wed, 19 Jun 2013, Ian Campbell wrote: > > On Tue, 2013-06-18 at 19:56 +0100, Stefano Stabellini wrote: > > > On Fri, 14 Jun 2013, Paul Durrant wrote: > > > > > -----Original Message----- > > > > > From: Paolo Bonzini [mailto:paolo.bonzini@gmail.com] On Behalf Of > Paolo > > > > > Bonzini > > > > > Sent: 14 June 2013 15:58 > > > > > To: Paul Durrant > > > > > Cc: Ian Campbell; Stefano Stabellini; qemu-devel@nongnu.org; xen- > > > > > devel@lists.xen.org > > > > > Subject: Re: [Xen-devel] [PATCH] Remove hardcoded xen-platform > device > > > > > initialization > > > > > > > > > > Il 14/06/2013 10:11, Paul Durrant ha scritto: > > > > > > I think we''re still going to need -M xenpv, I think; it''s quite > > > > > > distinct from pc. > > > > > > > > > > Of course! Even more: "-M xenpv" should be reused on ARM. > > > > > > > > > > > I guess we could use -M pc for HVM and gate the > > > > > > accel code as you suggest but, if that''s the way we''re going, it > > > > > > would seem more logical just to ditch the accel code for xenpv > > > > > > completely (assuming we can do all we need from the machine init) > and > > > > > > then use -M pc -accel=xen for HVM guests going forward. > > > > > > > > > > There is common code between pv and fv, and that one definitely > belongs > > > > > in xen_init. Most fv-only code probably should be in pc_init. The rest > > > > > should move to xen_init though, because it would apply just as well > for > > > > > example to Q35. It''s a bit ugly to have fv-only code there, but it''s > > > > > better than having a Xen-specific machine type. Xen/KVM/TCG > should be > > > > > as similar as possible at the QEMU level, any difference should be > > > > > handled in the toolstack. > > > > > > > > > > > But that does > > > > > > rather screw up my autodiscovery plans because I would not know, > for > > > > > > a given qemu binary, which machine type to use. > > > > > > > > > > There''s no need for that. 4.4 can just use "-M pc" unconditionally, > > > > > <=4.3 will just use "-M xenfv" unconditionally. > > > > > > > > > > > If I create a new > > > > > > xenfv-2.0 machine type though I *can* do auto discovery... in which > > > > > > case do we need the -accel=xen option at all? > > > > > > > > > > Yes. Please try not do things differently from other accelerators. > > > > > > > > > > > > > Ok. I guess we can have the ability to override the machine type in the > VM config, so you could still kick off an older qemu with a newer libxl - but it > sounds like the auto-discovery idea is a no-go then. > > > > > > xenfv-2.0 is a bad idea, like Paolo wrote, it should be possible to just > > > use -M pc for HVM guests and retain -M xenpv for pv guests. > > > > > > However it seems to me that we also need a way in libxl to find out > > > whether QEMU is new enough for us to be able to use -M pc. > > > We can''t just assume that users will be able to figure out the magic > > > rune they need to write in the VM config file to solve their VM crash at > > > boot problem. > > > > What crash at boot problem? > > If you start QEMU as device model on Xen with the wrong machine option > (for example -M pc on an old QEMU), QEMU would probably just abort > during initialization. > > > > > We could spawn an instance of QEMU just to figure out the QEMU > version > > > but we certainly cannot do that every time we start a new VM. > > > Once we figure out the QEMU version the first time we could write it to > > > xenstore so that the next time we don''t have to go through the same > > > process again. > > > > Due to the device_model_override we might need to make this per-path. > > You''d also likely need to store mtime or something in case qemu gets > > upgraded, although perhaps that is getting unnecessarily picky... > > I think of device_model_override as an option for developers. People > that use device_model_override can also override the QEMUMachine > version.Are you suggesting we allow a freeform -machine option in libxl, or are you suggesting they point device_model_override at a script which drops the -M argument and inserts their new choice before invoking qemu? Paul> I am more worried about your average user that gets a default broken > configuration on her favourite distro.