Am 05.10.2012 04:24, schrieb Alexander Graf: > > On 05.10.2012, at 04:17, Anthony Liguori wrote: > >> Alexander Graf <ag...@suse.de> writes: >> >>> On 03.10.2012, at 22:26, Peter Maydell wrote: >>> >>>> On 3 October 2012 21:01, Blue Swirl <blauwir...@gmail.com> wrote: >>>>> On Mon, Oct 1, 2012 at 4:20 PM, Anthony Liguori <anth...@codemonkey.ws> >>>>> wrote: >>>>>> Jan Kiszka <jan.kis...@siemens.com> writes: >>>>>>> + /* The default accelerator depends on the availability of KVM. >>>>>>> */ >>>>>>> + p = kvm_configured ? "kvm" : "tcg"; >>>>>>> } >>>> >>>>>> Blue/Aurelien, any objections? >>>>> >>>>> No, maybe a message could be printed that says that the default has >>>>> changed, for a few releases. >>>> >>>> I've lost track of the conversation, are we currently proposing >>>> the accelerator default to be "kvm" (as per the original patch >>>> you quote here) or "kvm:tcg" ? >>>> >>>> I'm not entirely sure which I prefer from an ARM perspective >>>> For some time to come and for a lot of targets (ie any target >>>> CPU except A15), having a default of "kvm" is going to cause >>>> existing working commandlines to stop working. [I expect that >>>> ARM-host qemu binaries will be built with CONFIG_KVM once ARM >>>> KVM support lands, but the same binary will be run on hosts >>>> without virtualization extensions.] On the other hand, perhaps >>>> there just aren't really very many people who run QEMU on >>>> ARM hosts, and so we can ignore them :-) >>> >>> We get similar problems on PPC. Take the following example: >>> >>> $ qemu-system-ppc -M mpc8544ds -kernel uImage -nographic >> >> But do you really expect people to do this? I have to believe that >> people running on PPC hardware and running qemu-system-ppc most likely >> want to do KVM... > > Sure. But we wouldn't be able to even tell them what went wrong, as we don't > have a negotiation mechanism right now that could tell user space "hey, the > CPU you selected is unknown to me".
Would it help to split out the cpu_model -> CPUClass lookup from cpu_ppc_init() to invoke a hook or inquire a field indicating KVM support? Andreas > > However, if during cpu init we could add such a check and then fall back to > tcg mode if accel=kvm:tcg with a warning, that'd be nice user experience. > > We could do the same for ARM. If you do -M beagle on an A15 KVM enabled > machine, you would still be able to do so, but KVM tells you it can't emulate > an A8 right now. And if in the future KVM learns how to expose an A8 on A15, > we could just not bail out and things would magically work. > > Apart from that, I like the idea of kvm:tcg with a warning as the default for > qemu. We should still have a qemu-kvm binary in the distro that does > accel=kvm so people don't accidentally fall back to tcg mode. > > > Alex > -- SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg, Germany GF: Jeff Hawn, Jennifer Guild, Felix Imendörffer; HRB 16746 AG Nürnberg