On Thu, Sep 18, 2014 at 03:36:43PM +0200, Paolo Bonzini wrote:
> We're talking about the case where the field is not reserved anymore and
> we _know_ that the vendor has just decided to grow the bitfield that
> precedes it.
We're talking about the case where you assumed that a reserved bit is 0
which is an unsafe assumption, the least.
> As soon as we know that the field is not reserved anymore, we
> obviously rely on reserved bits being zero in older processors, and in
> future processors from other vendors.
Again, this is an unsafe assumption.
> The trivial example is feature bits like XSAVE. We query them all the
> time without checking the family when they were first introduced,
> don't we?
The feature bits would obviously be 0 if features are not supported.
However, even there
"16 - Reserved - Do not count on the value."
I'm quoting Intel's CPUID doc 241618-037 from 2011 (there might be a
newer one though), the CPUID(1).ECX description.
--
Regards/Gruss,
Boris.
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