RE: [Xen-devel] [PATCH] pvcpuid: mask TSC invariant bit for various circumstances

[Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>]

> > Frankly, I'm  not very thrilled with the hack-y pvcpuid
> > requirement anyway.  It seemed a nice architected interface
> > if cpuid would work, but would be too easy for a well-intentioned
> > app developer to use cpuid instead of pvcpuid and make
> > incorrect assumptions that would result in app breakage.
> 
> What's hacky about it? It'll probably get used in any 
> 'solution' in some
> way, e.g., to probe for wacky new userland hypercall 
> interfaces. It's our
> standard interface to probe for features and information 
> direct from the
> hypervisor.

No offense intended.  The hacky part is my attempt for
the same cpuid bit (Invariant TSC) to have slightly
different interpretations depending on whether it is
physical (cpuid) or virtualized (pvcpuid); trying to
overload the definition of a single cpuid bit this way seemed
on closer thought fraught with potential confusion, especially
since it isn't a complete solution (for testing rdtsc
"safeness") anyway.

On the other hand, short of exposing a larger set
of "userland hypercalls" (whatever their underlying
implementation), I'm not seeing any better
way to provide this information to an app.

So are you suggesting that the best mechanism for
"userland hypercalls" is special reserved cpuid leaves?

Also, any comments on the meat of my last reply?

Thanks,
Dan

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