Re: [PATCH v6 08/42] x86/sev-es: initialize sev_status/features within #VC handler

On Thu, Oct 21, 2021 at 06:47:50PM +0100, Dr. David Alan Gilbert wrote:

Hang on, I think it's perfectly fine for it to clear that bit - it just
gets caught if it *sets* it (i.e. claims to be a chip unaffected by the
bug).

i.e. if guestval=0 then (GustVal & whatever) == GuestVal
  fine

?

Bah, ofc. The name of the bit is NullSelectorClearsBase - so when it is
clear, we will note we're affected, as that patch does:

+       /*
+        * CPUID bit above wasn't set. If this kernel is still running
+        * as a HV guest, then the HV has decided not to advertize
+        * that CPUID bit for whatever reason.  For example, one
+        * member of the migration pool might be vulnerable.  Which
+        * means, the bug is present: set the BUG flag and return.
+        */
+       if (cpu_has(c, X86_FEATURE_HYPERVISOR)) {
+               set_cpu_bug(c, X86_BUG_NULL_SEG);
+               return;
+       }

I have managed to flip the meaning in my mind.

Ok, that makes more sense.

Thx.

-- 
Regards/Gruss,
    Boris.

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