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

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Message-ID: <86ttqpm8lg.wl-maz@kernel.org>
Date:   Tue, 17 Oct 2023 19:01:31 +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 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):

bash-5.1# echo 0 >/sys/devices/system/cpu/cpu2/online 
[   51.367137] psci: CPU2 killed (polled 0 ms)
bash-5.1# echo 1 >/sys/devices/system/cpu/cpu2/online 
[   56.703092] Unable to handle kernel write to read-only memory at virtual address ffff800080d63db0
[   56.703494] Mem abort info:
[   56.703627]   ESR = 0x000000009600004f
[   56.703799]   EC = 0x25: DABT (current EL), IL = 32 bits
[   56.704052]   SET = 0, FnV = 0
[   56.704196]   EA = 0, S1PTW = 0
[   56.704340]   FSC = 0x0f: level 3 permission fault
[   56.704575] Data abort info:
[   56.704715]   ISV = 0, ISS = 0x0000004f, ISS2 = 0x00000000
[   56.704961]   CM = 0, WnR = 1, TnD = 0, TagAccess = 0
[   56.705195]   GCS = 0, Overlay = 0, DirtyBit = 0, Xs = 0
[   56.705435] swapper pgtable: 4k pages, 48-bit VAs, pgdp=000000008142e000
[   56.705742] [ffff800080d63db0] pgd=10000000dbfff003, p4d=10000000dbfff003, pud=10000000dbffe003, pmd=10000000dbffc003, pte=0060000080d63783
[   56.706310] Internal error: Oops: 000000009600004f [#1] PREEMPT SMP
[   56.706591] Modules linked in:
[   56.706741] CPU: 2 PID: 0 Comm: swapper/2 Not tainted 6.6.0-rc1-00007-g805cc7d30885-dirty #1702
[   56.707145] Hardware name: linux,dummy-virt (DT)
[   56.707359] pstate: 800001c5 (Nzcv dAIF -PAN -UAO -TCO -DIT -SSBS BTYPE=--)
[   56.707676] pc : cpu_enable_cnp+0x24/0x60
[   56.707873] lr : verify_local_cpu_caps+0x6c/0x140
[   56.708046] sp : ffff800082103e10
[   56.708201] x29: ffff800082103e10 x28: 0000000000000000 x27: 0000000000000000
[   56.708517] x26: 0000000000000000 x25: 0000000000000000 x24: 0000000000000000
[   56.708837] x23: ffff800081eec540 x22: 0000000000000003 x21: ffff8000813945f0
[   56.709166] x20: ffff800081394358 x19: ffff800080d63db0 x18: 0000000000000000
[   56.709488] x17: 000000040044ffff x16: 00500073b5503510 x15: 0000000000000000
[   56.709805] x14: 0000000000000000 x13: 0000000000000000 x12: 0000000000000000
[   56.710100] x11: 0000000000000000 x10: 0000000000000000 x9 : ffff8000800273ac
[   56.710416] x8 : ffff80008003b640 x7 : 0000000000000000 x6 : 0000000000000000
[   56.710738] x5 : 0000000000000000 x4 : ffff800081394000 x3 : ffff800081394000
[   56.711070] x2 : ffff800081241ef8 x1 : ffff800080027218 x0 : ffff800080d63db0
[   56.711396] Call trace:
[   56.711512]  cpu_enable_cnp+0x24/0x60
[   56.711684]  verify_local_cpu_caps+0x6c/0x140
[   56.711885]  verify_local_cpu_capabilities+0x1c/0x198
[   56.712112]  check_local_cpu_capabilities+0x28/0x50
[   56.712337]  secondary_start_kernel+0xb4/0x1a0
[   56.712546]  __secondary_switched+0xb8/0xc0
[   56.712739] Code: 910003fd b0009b63 d00090c2 913be042 (f9000002) 
[   56.713013] ---[ end trace 0000000000000000 ]---
[   56.713224] Kernel panic - not syncing: Attempted to kill the idle task!
[   56.713518] SMP: stopping secondary CPUs
[   56.713752] Kernel Offset: disabled
[   56.713908] CPU features: 0x00000000,50024d43,1947f723
[   56.714144] Memory Limit: none
[   56.714292] ---[ end Kernel panic - not syncing: Attempted to kill the idle task! ]---

Thanks,

	M.

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