Re: [PATCH 0/2] Viridian MSRs

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 16/10/13 11:12, Jan Beulich wrote:
>>>> On 15.10.13 at 20:12, Andrew Cooper <andrew.cooper3@citrix.com> wrote:
>> This set of two patches advertises 3 constant, read-only MSRs of timing
>> information to a viridian capable VM.
>>
>> There is an as-yet-unidentified issue when running Windows 8.1 / Server 2012r2
>> under Xen where it will periodically (1 in 10 attempt) appear to fall into 
>> an
>> idle loop rather than schedule userspace processes (such as failing to run a
>> login session).
>>
>> I am still investigating the underlying cause.  One possibility is an
>> interaction of TSC time calibration interacting poorly with the Xen 
>> scheduler.
>>
>> Unfortunately, attempting to divine what windows is unhappy about with its
>> environment is rather tricky (even a BSOD would be more useful than the
>> current symptoms), but providing these MSRs causes Windows to prefer rdtsc
>> over the HPET main counter as a source of time, and 'fixes' the above issue.
> I'm curious whether you would have put any consideration into
> the growing use of Hyper-V features when available - they had
> to play tricks in the past to avoid using them when in fact running
> on Xen. In particular in the case here I'm not certain your change
> won't interact badly with https://lkml.org/lkml/2013/9/3/417.
>
> Jan
>

On Xen, viridian extensions is still an opt-in feature using an hvm param.

I don't see how this would interact badly with that change?  If Linux or
indeed anything else is unable to tell the difference between running on
Xen and running on hyperV, that is a but in the guest, not a bug in Xen
for providing viridian according to the specification.

~Andrew