Re: [libvirt] [PATCH] target-i386: Disable CPUID_EXT_MONITOR when KVM is enabled
(CCing libvirt people)
On Tue, May 28, 2013 at 06:48:52PM +0200, Andreas Färber wrote:
Am 28.05.2013 18:46, schrieb Paolo Bonzini:
> Il 28/05/2013 18:34, Bandan Das ha scritto:
>> Eduardo Habkost <ehabkost(a)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.
I believe we can still consider the "vendor" field a special one: if
other components care about the TCG/KVM difference regarding the
"vendor" field, they can simply set "vendor" explicitly on the
command-line.
>> I can see Paolo's point though, having
>> a common definition probably makes sense too.
>
Paolo is convincing me that keeping the rest of the features exactly the
same on TCG and KVM modes (and making check/enforce work for TCG as
well) would simplify the logic a lot. This will add a little extra work
for libvirt, that will probably need to use "-cpu Opteron_G3,-monitor"
once it implements enforce-mode (to make sure the results really match
existing libvirt assumptions about the Opteron_G* models), but it is
probably worth it.
I will give it a try and send a proposal soon.
>>> (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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Eduardo