Re: [Qemu-devel] [PATCH] Introduce -accel command option.

[Alexander Graf]

On 16.11.2010, at 19:22, Anthony PERARD wrote:

> On Tue, 16 Nov 2010, Anthony Liguori wrote:
> 
>> On 11/16/2010 11:24 AM, Alexander Graf wrote:
>>> On 16.11.2010, at 18:20, Anthony Liguori wrote:
>>> 
>>> 
>>>> On 11/16/2010 10:59 AM, Alexander Graf wrote:
>>>> 
>>>>> 
>>>>>> Surely, only if it is running on a Xen Dom0. If you use -M xenpv on a KVM
>>>>>> host, then -M xenpv should imply -machine accel=kvm (ie it would be using
>>>>>> xenner)
>>>>>> 
>>>>>> 
>>>>> Actually, it should imply -machine accel=kvm,tcg :). Accelerators really 
>>>>> are not a machine property. In an ideal world, -M pc would just work with 
>>>>> xen hvm if -accel xen is given.
>>>>> 
>>>>> 
>>>> No, an accelerator is both a CPU selection and a machine characteristic.  
>>>> For KVM, we overload -cpu to modify both the KVM CPU and the TCG CPU both 
>>>> this won't work with accel=xen.  We probably shouldn't do this with KVM 
>>>> either because there's a significant different between trying to do cpuid 
>>>> masking with KVM and modifying the TCG cpu emulation support.
>>>> 
>>>> Both KVM and Xen have other impacts on the platform devices though.  KVM 
>>>> does not support SMM so it disables that in the i440fx.  KVM prefers to 
>>>> use it's own in-kernel local APIC (and IOAPIC).  That makes it a property 
>>>> of the machine.
>>>> 
>>> So you're saying the machine should define an accel mask of accels it 
>>> supports?
>>> 
>>> However all this ends up internally, giving the user an easy option to 
>>> choose accels would still be nice. Users don't use -device or -machine. 
>>> They want shortcuts :).
>>> 
>> 
>> User's want things to just work.
>> 
>> That's why -M xenpv should imply -machine accel=xen.
> 
> Actually, it works like that for qemu-xen, we just specify -M xenpv, and
> we don't have any --enable-xen or other -accel xen.

Yes, and that's exactly the behavior you'd get :). -accel would default to 
xen,kvm,tcg and thus -M xenpv on xen defaults to real xen pv code, -M xenpv on 
kvm would default to xenner w/ kvm and -M xenpv without any accelerator 
available would just fall back to xenner with tcg. Everyone is happy :).


Alex