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    SubjectRe: [PATCH 4/8] x86/traps: Demand-populate PASID MSR via #GP
    On Wed, Sep 22, 2021 at 02:33:09PM -0700, Dave Hansen wrote:
    > 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.

    Well, they won't get past the X86_FEATURE check anyway, so who cares.

    > 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. :)

    Instruction deoding makes it obvious you only consume your own #GP, the
    alternative is a comment that explains this reasoning. Having neither
    gets you confusion as per this thread.

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