linux-kernel - Re: [PATCH v2 3/3] arm64: cpufeature: Change 32-bit EL0 to display enabled cores

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Message-ID: <86pm1cm780.wl-maz@kernel.org>
Date:   Wed, 18 Oct 2023 13:43:27 +0100
From:   Marc Zyngier <maz@...nel.org>
To:     Jeremy Linton <jeremy.linton@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, catalin.marinas@....com,
        will@...nel.org, mark.rutland@....com, anshuman.khandual@....com,
        krisman@...e.de, broonie@...nel.org, james.morse@....com,
        ionela.voinescu@....com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 3/3] arm64: cpufeature: Change 32-bit EL0 to display enabled cores

On Tue, 17 Oct 2023 20:15:43 +0100,
Jeremy Linton <jeremy.linton@....com> wrote:
> 
> Hi,
> 
> On 10/17/23 13:01, Marc Zyngier wrote:
> > On Tue, 17 Oct 2023 06:23:22 +0100,
> > Jeremy Linton <jeremy.linton@....com> wrote:
> >> 
> >> Now that we have the ability to display the list of cores
> >> with a feature when it is selectivly enabled, lets display the
> >> cores enabled for 32-bit use at EL0.
> >> 
> >> Signed-off-by: Jeremy Linton <jeremy.linton@....com>
> >> ---
> >>   arch/arm64/kernel/cpufeature.c | 15 +++++++++++++--
> >>   1 file changed, 13 insertions(+), 2 deletions(-)
> >> 
> >> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> >> index b7b67bac0e60..512cbe446b41 100644
> >> --- a/arch/arm64/kernel/cpufeature.c
> >> +++ b/arch/arm64/kernel/cpufeature.c
> >> @@ -1533,8 +1533,17 @@ static bool has_32bit_el0(const struct arm64_cpu_capabilities *entry, int scope)
> >>   	if (!has_cpuid_feature(entry, scope))
> >>   		return allow_mismatched_32bit_el0;
> >>   -	if (scope == SCOPE_SYSTEM)
> >> -		pr_info("detected: 32-bit EL0 Support\n");
> >> +	if (scope == SCOPE_SYSTEM) {
> >> +		struct arm64_cpu_capabilities *has_32bit;
> >> +
> >> +		has_32bit = (struct arm64_cpu_capabilities *)entry;
> >> +
> >> +		has_32bit->cpus = system_32bit_el0_cpumask();
> > 
> > This seems rather dodgy. 'entry' comes from a static const array which
> > will, in all likelihood be mapped R/O pretty soon after the initial
> > CPU bringup. Try offlining/onlining a CPU and you should see a
> > firework similar to what I have below (I hacked the CnP property, but
> > that's no different from what you are doing):
> 
> Yes, dodgy is a good word. The other choices, maintain a mask just for
> the print or dump the static key and always use the cpu_32bit_el0_mask
> or some combination, weren't much better in the "ick" category. If
> anyone sees a better way I'm open to suggestion, although simply
> dropping this last patch is fine too.

An obvious choice would be to replace the 'cpus' cpumask with a
function that evaluates a cpumask stored separately.

	M.

-- 
Without deviation from the norm, progress is not possible.