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

[D Scott Phillips <scott@os.amperecomputing.com>]

Catalin Marinas <catalin.marinas@arm.com> writes:

> 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 ;).

I measured it at something like 20-30x the time of a read, though that
was in a tight loop of writing, so maybe the cost could be hidden some
by out-of-order execution. Are you thinking of moving the erratum
workaround back to the exit to user path?

> -- 
> Catalin

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