Re: [Qemu-devel] [PATCH 2/2] Expose tsc deadline timer cpuid to guest

[Jan Kiszka <jan.kiszka@siemens.com>]

On 2012-04-20 17:00, Eduardo Habkost wrote:
> On Fri, Apr 20, 2012 at 12:12:38PM +0200, Jan Kiszka wrote:
>> On 2012-04-19 22:03, Eduardo Habkost wrote:
>>> Jan/Avi: ping?
>>>
>>> I would like to get this ABI detail clarified so it can be implemented
>>> the right way on Qemu and KVM.
>>>
>>> My proposal is to simply add tsc-deadline to the data returned by
>>> GET_SUPPORTED_CPUID, making KVM_CAP_TSC_DEADLINE_TIMER unnecessary.
>>>
>>
>> IIRC, there were problems with this model to exclude that the feature
>> gets reported this way without ensuring that in-kernel irqchip is
>> actually activated. Please browse the discussions, it should be
>> documented there.
> 
> The flag was never added to GET_SUPPORTED_CPUID on any of the git
> repositories I have checked, and I don't see that being done anywhere on
> my KVM mailing list archives, either. So I don't see how we could have
> had issues with GET_SUPPORTED_CPUID, if it was never present on the
> code.
> 
> What was present on the code before the fix, was coded that
> unconditinally enabled the flag even if Qemu never asked for it, and
> that really was wrong.
> 
> GET_SUPPORTED_CPUID doesn't enable any flag: it just tells userspace
> that the hardware and KVM supports the feature (and, surprise, this is
> exactly what KVM_CAP_TSC_DEADLINE_TIMER means too, isn't it?). It's up
> to userspace to enable the CPUID bits according to user requirements and
> userspace capabilities (in other words: only when userspace knows it is
> safe because the in-kernel irqchip is enabled).
> 
> If the above is not what GET_SUPPORTED_CPUID means, I would like to get
> that clarified, so I can submit an updated to KVM API documentation.

Does old userspace filter out flags from GET_SUPPORTED_CPUID that it
does not understand?

Jan

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