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

[Catalin Marinas]

On Tue, Dec 21, 2021 at 12:10:08PM -0800, D Scott Phillips wrote:
> Catalin Marinas <catalin.marinas at 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?

No, just wondering whether we can avoid the read/check/write with
preemption disabled. Thread switches happen less often than the return
to user.

I'll probably take your current patch as a fix of Marc's commit. Waiting
a bit to see if Marc has any further comments.

-- 
Catalin