Re: [Xen-devel] Restoring FPU exception state

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 18.02.16 at 09:54, <Paul.Durrant@xxxxxxxxxx> wrote:
>>  -----Original Message-----
>> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
>> Sent: 18 February 2016 08:50
>> To: Paul Durrant; Kevin Tian
>> Cc: Andrew Cooper; David Vrabel; Feng Wu; xen-devel
>> Subject: RE: Restoring FPU exception state
>> 
>> >>> On 18.02.16 at 09:41, <Paul.Durrant@xxxxxxxxxx> wrote:
>> >>  -----Original Message-----
>> >> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
>> >> Sent: 18 February 2016 08:16
>> >> To: Kevin Tian
>> >> Cc: Andrew Cooper; David Vrabel; Paul Durrant; Feng Wu; xen-devel
>> >> Subject: RE: Restoring FPU exception state
>> >>
>> >> >>> On 18.02.16 at 07:30, <kevin.tian@xxxxxxxxx> wrote:
>> >> > Interesting. Let me also have a check internally whether there is other
>> >> > architectural alternative. BTW, not quite related. Could I think finally
>> >> > Xen may allow user to specify OS type as a general per-domain control,
>> >> > and then Xen can do free optimizations underlying based on that type?
>> >> > I don't expect we want to expose raw FPU related per-domain control
>> >> > since it's difficult for users to correctly set it up...
>> >>
>> >> So what if then, with us implying some kind of workaround here
>> >> from the OS type, someone set the OS type to Windows, and
>> >> subsequently MS decided to fix their issue?
>> >
>> > We already get full OS version information from Windows via the
>> > VIRIDIAN_MSR_GUEST_OS_ID MSR so it would be quite straightforward to
>> work
>> > around the issue for all Windows at the moment and then make the check
>> more
>> > specific if Microsoft choose to avoid the problem by some other means.
>> 
>> That's only if "viridian=1" in the guest config, isn't it?
> 
> Yes, but most people are going to run with that on for HVM guests;

That view may be pretty XenServer centric.

> in 
> general there's no penalty for doing so unless you're running some old and 
> buggy OS. If it's turned off then there's nothing Xen can do other than 
> continue to behave as it does now.
> 
>> Plus - how would
>> we know MS chose to fix their apparent issue?
>> 
> 
> Well either we'd hear about it or we can run tests on each new version of 
> Windows (it wouldn't be hard to add a specific test for this and add it 
> XenServer's automated suite). 

Well, if that also covers kernel updates for existing versions of
Windows.

But taking both together, defaulting the behavior based on the
Viridian flag is certainly an option, just that we'd need an override
both ways nevertheless.

Jan


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