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

[Andreas Färber]

Am 05.10.2012 04:24, schrieb Alexander Graf:
> 
> On 05.10.2012, at 04:17, Anthony Liguori wrote:
> 
>> 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...
> 
> 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
> 


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