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

From: Alexander Graf <agraf@suse.de>
To: "Andreas Färber" <afaerber@suse.de>
Cc: Anthony Liguori <anthony@codemonkey.ws>,
	Peter Maydell <peter.maydell@linaro.org>,
	Blue Swirl <blauwirbel@gmail.com>, kvm <kvm@vger.kernel.org>,
	Scott Moser <smoser@ubuntu.com>,
	Jan Kiszka <jan.kiszka@siemens.com>,
	Marcelo Tosatti <mtosatti@redhat.com>,
	Michael Tokarev <mjt@tls.msk.ru>,
	qemu-devel <qemu-devel@nongnu.org>, Avi Kivity <avi@redhat.com>,
	Cole Robinson <crobinso@redhat.com>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Aurelien Jarno <aurelien@aurel32.net>
Subject: Re: [Qemu-devel] [PATCH] kvm: Set default accelerator to "kvm" if the host supports it
Date: Mon, 8 Oct 2012 16:08:30 +0200	[thread overview]
Message-ID: <10EEE983-5224-4236-9800-1EA7A8A6024A@suse.de> (raw)
In-Reply-To: <5072DD1C.9060808@suse.de>

On 08.10.2012, at 16:03, Andreas Färber wrote:

> Am 05.10.2012 04:24, schrieb Alexander Graf:
>> 
>> On 05.10.2012, at 04:17, Anthony Liguori wrote:
>> 
>>> Alexander Graf <agraf@suse.de> writes:
>>> 
>>>> 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
>>> 
>>> 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?

Well, we need to basically determine whether KVM is enabled only after cpu creation of the machine file.

Alex