Re: [Qemu-devel] [PATCH] kvm: Set default accelerator to "kvm" if the host supports it

[Alexander Graf <agraf@suse.de>]

On 03.10.2012, at 22:26, Peter Maydell wrote:

> On 3 October 2012 21:01, Blue Swirl <blauwirbel@gmail.com> wrote:
>> On Mon, Oct 1, 2012 at 4:20 PM, Anthony Liguori <anthony@codemonkey.ws> wrote:
>>> Jan Kiszka <jan.kiszka@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

That command line would work just fine if you run it on a G5 today. However, if we switch to accel=kvm:tcg, it will stop working, because kvm on the G5 can not virtualize an e500 CPU.

I don't know any good way around that one either the way the code is layered today. We only know the type of CPU we want to create after we made the decision whether we do KVM or not.


Alex