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

From: Paolo Bonzini
Subject: Re: [Qemu-devel] [PATCH] target-i386: Disable CPUID_EXT_MONITOR when KVM is enabled
Date: Tue, 28 May 2013 18:46:24 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130514 Thunderbird/17.0.6

Il 28/05/2013 18:34, Bandan Das ha scritto:
> Eduardo Habkost <address@hidden> 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?

> 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.


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