Re: [Xen-devel] [PATCH 2/2] libxl: fix osvm cpuid flag

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 28/06/17 18:51, Wei Liu wrote:
> On Wed, Jun 28, 2017 at 05:20:38PM +0100, Andrew Cooper wrote:
>> On 28/06/17 17:14, Marek Marczykowski-Górecki wrote:
>>> On Wed, Jun 28, 2017 at 11:16:33AM +0100, Andrew Cooper wrote:
>>>> On 28/06/17 07:09, Jan Beulich wrote:
>>>>>>>> Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx> 06/28/17 
>>>>>>>> 3:09 AM >>>
>>>>>> It's bit 9 not 10 (which is ibs).
>>>>> Indeed, but shouldn't it rather be removed? We don't expose it from the
>>>>> hypervisor at all anymore:
>>>>>
>>>>> XEN_CPUFEATURE(OSVW,          3*32+ 9) /*   OS Visible Workaround */
>>>>>
>>>>> (note the absence of any marker character immediately inside the comment).
>>>> I don't believe we have ever actually offered OSVW to guests, despite
>>>> the pretence of being able to.  ISTR it was always clobbered before
>>>> being given to a guest.
>>>>
>>>> Having said that, we should be advertising OSVW.  It's entire purpose is
>>>> to tell the OS that there is something it can do to manually work round
>>>> a specific erratum.  OTOH, making this migrate safe is liable to be very
>>>> complicated...
>>> I don't have opinion on either approach here, but the current state is
>>> clearly wrong. You've got two versions of the patch, choose one ;)
>>>
>> I'd prefer this version of the patch, so it doesn't suddenly remove a
>> piece of libxl API, but it is up to Wei / Ian at the end of the day.
> Keeping it is better. It doesn't really make any difference to the guest
> but some tools might complain if we remove it.
>
>> One option which was being discussed in the context of my CPUID (part 3)
>> work was to automatically this list of keywords.
>>
> "generate"?

Oops.  Yes.

> And yes, I fully support this work. ;-)

It's not a fully-formed idea yet.  It wouldn't easily deal with
duplicate names, and won't deal with CPUID content other than the
feature flags known to Xen at build time.

I eventually want to be able to express things like cpuid="pmu=3" to
enable vPMU emulating version 3.

~Andrew

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