Re: [RFC PATCH 1/2] Hyper-H reference counter

Vadim Rozenfeld
2013-05-20T10:25:23Z

On Mon, 2013-05-20 at 12:25 +0300, Gleb Natapov wrote:
> On Mon, May 20, 2013 at 10:56:22AM +0200, Paolo Bonzini wrote:
> > In any case, this patch needs more comments and a better commit message.
> > Microsoft docs are decent, but there are several non-obvious points in
> > how the patches were done, and they need to be documented.
> I wish you were right about Microsoft docs :) So in Hyper-V spec they
> say:
>
> Special value of 0xFFFFFFFF is used to indicate that this facility is no
> longer a reliable source of reference time and the virtual machine must
> fall back to a different source (for example, the virtual PM timer).
>
> May be they really mean "virtual PM timer" here and reference counter is
> not considered as a fall back source, but this is not what we want.

As far as I know, you cannot fall back from iTSC to PMTimer or HPET,
but you can fallback to reference counters.

>
> On the other hand in API specification [1] they have:
>
> #define HV_REFERENCE_TSC_SEQUENCE_INVALID (0x00000000)
>
> which is not even documented in hyper-v spec. Actually 0 is specified as
> valid value there. Go figure.
>
> [1] http://msdn.microsoft.com/en-us/library/windows/hardware/ff540244%28v=vs.85%29.aspx
>
> --
> Gleb.