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

[Alexander Graf]

On 16.11.2010, at 18:27, 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.

No, it should imply -machine accel=xen,kvm,tcg. Which should be the default 
anyways. For machines that don't work with specific accels, I'd agree that a 
machine should be able to mask those out. But in general, we'll always get back 
to defaulting to xen,kvm,tcg.

> A user should never have to specify and accelerator option IMHO.

If things don't work out, he should be able to do things like -no-kvm for bug 
hunting. In normal use cases, I tend to agree. Things should just work 
automatically.


Alex