Re: [PATCH] x86/split_lock: Avoid runtime reads of the TEST_CTRL MSR

[Andy Lutomirski]

On Wed, Feb 5, 2020 at 4:49 PM Luck, Tony <tony.luck@xxxxxxxxx> wrote:
>
> In a context switch from a task that is detecting split locks
> to one that is not (or vice versa) we need to update the TEST_CTRL
> MSR. Currently this is done with the common sequence:
>         read the MSR
>         flip the bit
>         write the MSR
> in order to avoid changing the value of any reserved bits in the MSR.
>
> Cache the value of the TEST_CTRL MSR when we read it during initialization
> so we can avoid an expensive RDMSR instruction during context switch.

If something else that is per-cpu-ish gets added to the MSR in the
future, I will personally make fun of you for not making this percpu.