On 9/22/21 2:11 PM, Peter Zijlstra wrote:
>>> +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?
To reiterate: on systems with no X86_FEATURE_ENQCMD, there is basically
no additional overhead. It isn't worth doing decoding there.
On systems with X86_FEATURE_ENQCMD, but where it is unused, the #GP
handler gets some new overhead on every #GP. Basically:
> + pasid = current->mm->pasid;
> + if (pasid == PASID_DISABLED)
> + return false;
That's still pretty cheap. Probably not worth doing decoding there either.
So, that leaves us with if you are:
1. On system with X86_FEATURE_ENQCMD
2. In a process/mm that has an allocated pasid
3. Your *task* does not have the MSR set
4. You get a #GP for some other reason
Then, you'll do this double-#GP dance.
So, instruction decoding could absolutely be added between steps 3 and
4. It would absolutely save doing the double-#GP in cases where 1/2/3
are met. But, I wouldn't move it up above and of the 1/2/3 checks
because they're way cheaper than instruction decoding.
In the end, it didn't seem worth it to me to be optimizing a relatively
rare path which 99% of the time ends up in a crash.
If you want instruction decoding in here, though, just say the word. :)
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