Re: [libvirt] [Qemu-devel] ARM KVM GICv3 Support

On Tue, Feb 02, 2016 at 12:10:10PM +0000, Daniel P. Berrange wrote: > On Tue, Feb 02, 2016 at 12:49:33PM +0100, Christoffer Dall wrote: > > On Fri, Jan 22, 2016 at 02:44:32PM +0000, Daniel P. Berrange wrote: > > > On Wed, Jan 06, 2016 at 01:30:16PM +0000, Peter Maydell wrote: > > > > On 6 January 2016 at 12:49, Andrea Bolognani <abologna(a)redhat.com&gt; wrote: > > > > > That's correct, having a QMP command that lists the values gic-version > > > > > can have on the current host would be just great. > > > > > > > > > > If we had that, we could validate the GIC version chosen for a guest, > > > > > and expose it in the capabilities XML so that higher-level tools can > > > > > provide a list of choices to the user. > > > > > > > > > > Please note that this QMP command would have to work regardless of the > > > > > machine type selected on QEMU's command line, because libvirt always > > > > > runs a QEMU binary with '-M none' when probing its capabilities. > > > > > > > > On the other hand, if you don't tell us the machine type you care > > > > about then we can't tell you: > > > > (a) "this machine type doesn't support setting this property at all" > > > > (which applies to machines like vexpress-a15 which you can use with > > > > KVM on 32-bit hosts, and of course also to all the non-KVM models) > > > > > > We have just recently merged support for registering properties against > > > classes instead of object instances. There is also a proposed command > > > to allow querying the list of properties against a class > > > > > > https://lists.gnu.org/archive/html/qemu-devel/2016-01/msg04348.html > > > > > > So if we now update the machine types to register their properties against > > > the class instead of object, then we can query what properties are present > > > against each machine type, while still using '-M none'. > > > > > > > (b) "this machine type only supports GIC versions X and Y even if the > > > > host supports more" (this is currently only hypothetical, though, > > > > since we only have the property on 'virt'. it would only happen > > > > if in the future we needed something other than '2' or '3' or > > > > 'host' I think.) > > > > > > Our introspection support in QOM only allows us to say that a property > > > is a particular type (int / enum / str / whatever). We don't have any > > > way to expose info about what subset of possible values for a type are > > > permitted. So I don't see any near term way to inform apps that the > > > gic property accepts values x, y and but not z. This actually doesn't matter for the v2 vs. v3 case. The gic-version property doesn't exist at all for v2-only QEMU. Although maybe the gic-version property should be reworked. Instead of gic-version=<highest-supported>, we could create one boolean property per version supported, i.e. gicv2, gicv3, gicv4... > > > > > > IMHO, we shouldn't try to overthink this. Libvirt can query the host > > > to find out what GIC versions are supported and default to using the > > > most recent version, on the basis that people are likely to have a > > > matching QEMU. We can just rely on QEMU to report error if we pass > > > it a version it doesn't support and not try to pre-emptively check > > > before launch. The key is just getting the default right IMHO. > > > > > This sounds fine to me. > > > > However, not being familiar with the internals of libvirt I really can't > > just what the right implementation approach here is. > > > > As I understand we need to either: > > > > 1) Add a QMP command that lets you ask for -M virt, if GIC version X > > is supported > > > > 2) Just implement something in libvirt that checks what the kernel > > supports directly via the well-defined KVM interface and chooses > > the highest supported version per default. > > > > To me it sounds like we should just go ahead with (2) and document > > somehwere that if you get an error, you need to either manually change > > the gic version setting in your VM definition file or upgrade QEMU. > > > > Can someone with libvirt authority please advice whether (1) or (2) is > > what we need to do? > > IMHO we should just go for 2. I'm not familiar enough with libvirt, nor the use of QMP, to really argue one way or another, but I find it a bit strange that we'd prefer libvirt to query two entities over one. And, why should the libvirt installed on a particular host prefer gicv3 as the default, just because KVM supports it, even when QEMU does not?

The default will fail every time until QEMU is upgraded, which may not be necessary/desired. Finally, I thought we were trying to get away from relying on QEMU's error messages to make any sort of decisions.

I don't know what else libvirt queries directly from KVM, but IMO, it should be a long-term goal to stop doing it, not add to more. Besides libvirt then properly selecting defaults that both KVM and QEMU support, it would allow /dev/kvm to have QEMU-only selinux labels applied.