Re: [Qemu-devel] [PATCH] target-i386: Disable CPUID_EXT_MONITOR when KVM is enabled

["Andreas Färber" <afaerber@suse.de>]

Am 28.05.2013 18:46, schrieb Paolo Bonzini:
> Il 28/05/2013 18:34, Bandan Das ha scritto:
>> Eduardo Habkost <ehabkost@redhat.com> writes:
>>
>>> On Mon, May 27, 2013 at 02:21:36PM +0200, Paolo Bonzini wrote:
>>>> Il 27/05/2013 14:09, Eduardo Habkost ha scritto:
>>>>> On Sat, May 25, 2013 at 08:25:49AM +0200, Paolo Bonzini wrote:
>>>>>> Il 25/05/2013 03:21, Bandan Das ha scritto:
>>>>>>> There is one user-visible effect: "-cpu ...,enforce" will stop failing
>>>>>>> because of missing KVM support for CPUID_EXT_MONITOR. But that's exactly
>>>>>>> the point: there's no point in having CPU model definitions that would
>>>>>>> never work as-is with neither TCG or KVM. This patch is changing the
>>>>>>> meaning of (e.g.) "-machine ...,accel=kvm -cpu Opteron_G3" to match what
>>>>>>> was already happening in practice.
>>>>>>
>>>>>> But then -cpu Opteron_G3 does not match a "real" Opteron G3.  Is it
>>>>>> worth it?
>>>>>
>>>>> No models match a "real" CPU this way, because neither TCG or KVM
>>>>> support all features supported by a real CPU. I ask the opposite
>>>>> question: is it worth maintaining an "accurate" CPU model definition
>>>>> that would never work without feature-bit tweaking in the command-line?
>>>>
>>>> It would work with TCG.  Changing TCG to KVM should not change hardware
>>>> if you use "-cpu ...,enforce", so it is right that it fails when
>>>> starting with KVM.
>>>>
>>>
>>> Changing between KVM and TCG _does_ change hardware, today (with or
>>> without check/enforce). All CPU models on TCG have features not
>>> supported by TCG automatically removed. See the "if (!kvm_enabled())"
>>> block at x86_cpu_realizefn().
>>
>> Yes, this is exactly why I was inclined to remove the monitor flag. 
>> We already have uses of kvm_enabled() to set (or remove) kvm specific stuff,
>> and this change is no different.
> 
> Do any of these affect something that is part of x86_def_t?

The vendor comes to mind.

Andreas

>> I can see Paolo's point though, having 
>> a common definition probably makes sense too.
> 
>>> (That's why I argue that we need separate classes/names for TCG and KVM
>>> modes. Otherwise our predefined models get less useful as they will
>>> require low-level feature-bit fiddling on the libvirt side to make them
>>> work as expected.)
>>
>> Agreed. From a user's perspective, I think the more a CPU model "just works",
>> whether it's KVM or TCG, the better.
> 
> Yes, that's right.  But I think extending the same expectation to "-cpu
> ...,enforce" is not necessary, and perhaps even wrong for "-cpu
> ...,check" since it's only a warning rather than a fatal error.
> 
> Paolo
> 


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