Re: [PATCH 0/1] x86/split_lock: check split lock feature on initialization

[Thomas Gleixner]

Benjamin Lamowski <benjamin.lamowski@xxxxxxxxxxxxxxx> writes:
> During regression testing of our hypervisor[1] with the current git tip,
> we got writes to the TEST_CTRL MSR on hardware that does not support
> split lock detection. While the original split_lock implementation does
> not exhibit this behavior, the reworked initialization from
> dbaba47085b0c unconditionally calls split_lock_verify_msr() from
> split_lock_init().
>
> After the elaborate checks in cpu_set_core_cap_bits() this seems like an
> oversight. The following simple patch fixes our regression by checking
> for X86_FEATURE_SPLIT_LOCK_DETECT before accessing the TEST_CTRL MSR.

No. It's not an oversight, it's a simplification and it's perfectly
legit. rdsmrl_safe() on a unimplemented MSR results in a #GP which is
caught and fixed up. Nothing to see here.

Thanks,

        tglx