RE: [PATCH 4/8] x86/traps: Demand-populate PASID MSR via #GP

["Luck, Tony" <tony.luck@intel.com>]

>> > +static bool fixup_pasid_exception(void)
>> > +{
>> > +	if (!cpu_feature_enabled(X86_FEATURE_ENQCMD))
>> > +		return false;
>> > +
>> > +	return __fixup_pasid_exception();
>> > +}
>
> That is, shouldn't the above at the very least decode the instruction
> causing the #GP and check it's this ENQCMD thing?

It can't reliably do that because some other thread in the process may
have re-written the memory that regs->ip points at (bizarre case, but
I think Dave Hansen brought it up).

So it would just add extra code, and still only be a hint.

Without the check this sequence is possible:

1) Process binds an accelerator (so mm->pasid is set)
2) Task in the process executes something other than ENQCMD that gets a #GP
3) Kernel says "Oh, mm->pasid is set, I'll initialize the IA32_PASID MSR to see if that fixes it"
4) Nope. Re-executing the instruction at step #2 just gives another #GP
5) Kernel says "I already set IA32_PASID, so this must be something else ... do regular #GP actions"

Now if the task catches the signal that results from step #5 and avoids termination, it will have
IA32_PASID set ... but to the right value should it go on to actually execute ENQCMD at some
future point.

So the corner case from not knowing whether this #GP was from ENQCMD or not is harmless.

-Tony