On Tue, 2016-01-05 at 18:14 +0000, Peter Maydell wrote:
Why can libvirt not just run the tool and parse its output? That's what I understood was done for other things, but perhaps I misunderstood. It's the other way around: the tool uses libvirt to implement its checks. Running a command and parsing its output makes sense when the binary (as is the case for QEMU) is developed by a third party; if you control both pieces, it's usually better to stick to C and avoid spawning extra processes. That argument suggests libvirt should just go ahead and do the ioctls to check for GICv2/v3 support, rather than spawning a separate QEMU binary to do the check for it ?
libvirt already has to spawn QEMU and run QMP commands to probe for dozens of other capabilities, so we have very robust code to do that. Most importantly, as I wrote in a previous mail, if the check is done by QEMU we remove the risk of it and libvirt ever disagreeing.
I realized the information is already sort of exposed by QEMU: $ qemu-system-aarch64 -machine virt,? ... virt.gic-version=string (Set GIC version. Valid values are 2, 3 and host) ... Not the best format to parse, but the information is there. And we already have some code to extract such parameters from QEMU using QMP's query-command-line-options command![1] Note that helpstring will tell you whether the qemu you have supports the machine parameter, but not whether the host CPU supports GICv2 or 3 (or both) -- it's just a fixed string.
Right, thanks for pointing that out :) Cheers. -- Andrea Bolognani Software Engineer - Virtualization Team