Messages in this thread | | | Date | Fri, 19 Sep 2014 09:58:15 +0200 | From | Borislav Petkov <> | Subject | Re: [RESEND PATCH 1/3] x86: Adding structs to reflect cpuid fields |
| |
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. --
| |