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

[Anthony Liguori]

Alexander Graf <address@hidden> writes:

> On 03.10.2012, at 22:26, Peter Maydell wrote:
>
>> On 3 October 2012 21:01, Blue Swirl <address@hidden> wrote:
>>> On Mon, Oct 1, 2012 at 4:20 PM, Anthony Liguori <address@hidden> wrote:
>>>> Jan Kiszka <address@hidden> 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...

Kernel development seems unlikely...  The folks that I know doing PPC
kernel development with QEMU still qemu-system-ppc on x86.

Regards,

Anthony Liguori

>
> 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