[PATCH v5] arm64: errata: Fix exec handling in erratum 1418040 workaround

[Catalin Marinas]

On Mon, Dec 20, 2021 at 03:41:14PM -0800, D Scott Phillips wrote:
> The erratum 1418040 workaround enables CNTVCT_EL1 access trapping in EL0
> when executing compat threads. The workaround is applied when switching
> between tasks, but the need for the workaround could also change at an
> exec(), when a non-compat task execs a compat binary or vice versa. Apply
> the workaround in arch_setup_new_exec().
> 
> This leaves a small window of time between SET_PERSONALITY and
> arch_setup_new_exec where preemption could occur and confuse the old
> workaround logic that compares TIF_32BIT between prev and next. Instead, we
> can just read cntkctl to make sure it's in the state that the next task
> needs. I measured cntkctl read time to be about the same as a mov from a
> general-purpose register on N1. Update the workaround logic to examine the
> current value of cntkctl instead of the previous task's compat state.

The patch looks fine to me but I was wondering what the cost of writing
CNTKCTL_EL1 is, compared to a read. If it turns out to be negligible, we
can simplify this patch further ;).

-- 
Catalin