Re: [RFC PATCH v2 4/4] arm64: Export id_aar64fpr0 via sysfs

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Mon, Oct 26, 2020 at 07:02:50PM +0000, Qais Yousef wrote:
> On 10/22/20 15:55, Greg Kroah-Hartman wrote:
> > On Thu, Oct 22, 2020 at 02:47:52PM +0100, Qais Yousef wrote:
> > > On 10/21/20 15:41, Will Deacon wrote:
> > > > > We already expose MIDR and REVIDR via the current sysfs interface. We
> > > > > can expand it to include _all_ the other ID_* regs currently available
> > > > > to user via the MRS emulation and we won't have to debate what a new
> > > > > interface would look like. The MRS emulation and the sysfs info should
> > > > > probably match, though that means we need to expose the
> > > > > ID_AA64PFR0_EL1.EL0 field which we currently don't.
> > > > > 
> > > > > I do agree that an AArch32 cpumask is an easier option both from the
> > > > > kernel implementation perspective and from the application usability
> > > > > one, though not as easy as automatic task placement by the scheduler (my
> > > > > first preference, followed by the id_* regs and the aarch32 mask, though
> > > > > not a strong preference for any).
> > > > 
> > > > If a cpumask is easier to implement and easier to use, then I think that's
> > > > what we should do. It's also then dead easy to disable if necessary by
> > > > just returning 0. The only alternative I would prefer is not having to
> > > > expose this information altogether, but I'm not sure that figuring this
> > > > out from MIDR/REVIDR alone is reliable.
> > > 
> > > So the mask idea is about adding a new
> > > 
> > > 	/sys/devices/system/cpu/aarch32_cpus
> > > 
> > > ?
> > 
> > Is this a file, a directory, or what?  What's the contents?
> > 
> > Without any of that, I have no idea if it's "ok" or not...
> 
> Hopefully the below patch explains better. Note that I added the new attribute
> to driver/base/cpu.c, but assuming we will still want to go down this route, we
> will need a generic way for archs to add their attributes to
> /sys/devices/system/cpu/.
> 
> Something like having a special define for archs to append their own
> attributes list
> 
> 	#define SYSFS_SYSTEM_CPU_ARCH_ATTRIBUTES
> 
> Or probably there's a way to add this file (attribute) dynamically from arch
> code that I just didn't figure out how to do yet.

Please do that, sysfs files should not be present when the information
is not needed from them.  Look at the is_visible() callback for the
attribute for how to do it.

> 
> Thanks
> 
> --
> Qais Yousef
> 
> 
> ---------->8------------
> 
> >From 96dfdfdacb2a26a60ba19051e8c72e839eb5408b Mon Sep 17 00:00:00 2001
> From: Qais Yousef <qais.yousef@arm.com>
> Date: Mon, 26 Oct 2020 16:33:32 +0000
> Subject: [PATCH] arm64: export aarch32_online mask in sysfs
> 
> This patch to be applied on top of arm64 Asymmetric AArch32 support.
> 
> It explores the option of exporting the AArch32 capable cpus as a mask
> on sysfs.
> 
> This is to help drive the discussion on the API before sending the next
> version which I yet to address some of the review comments.
> 
> The output looks like:
> 
> 	# cat /sys/devices/system/cpu/aarch32_online
> 	0-5
> 
> Signed-off-by: Qais Yousef <qais.yousef@arm.com>
> ---
>  Documentation/ABI/testing/sysfs-devices-system-cpu |  7 +++++++
>  arch/arm64/include/asm/cpufeature.h                |  2 ++
>  arch/arm64/kernel/cpufeature.c                     |  8 ++++++++
>  drivers/base/cpu.c                                 | 12 ++++++++++++
>  4 files changed, 29 insertions(+)
> 
> diff --git a/Documentation/ABI/testing/sysfs-devices-system-cpu b/Documentation/ABI/testing/sysfs-devices-system-cpu
> index b555df825447..9ccb5c3f5ee3 100644
> --- a/Documentation/ABI/testing/sysfs-devices-system-cpu
> +++ b/Documentation/ABI/testing/sysfs-devices-system-cpu
> @@ -36,6 +36,13 @@ Description:	CPU topology files that describe kernel limits related to
>  
>  		See Documentation/admin-guide/cputopology.rst for more information.
>  
> +What:		/sys/devices/system/cpu/aarch32_online
> +Date:		October 2020
> +Contact:	Linux ARM Kernel Mailing list <linux-arm-kernel@lists.infradead.org>
> +Description:	CPU topology file that describes which cpus support AArch32 at
> +		EL0. Only available on arm64.
> +
> +		The value is updated when a cpu becomes online then sticks.

What does "then sticks" mean?



>  
>  What:		/sys/devices/system/cpu/probe
>  		/sys/devices/system/cpu/release
> diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
> index 2b87f17b2bd4..edd18002ad81 100644
> --- a/arch/arm64/include/asm/cpufeature.h
> +++ b/arch/arm64/include/asm/cpufeature.h
> @@ -380,6 +380,8 @@ cpucap_multi_entry_cap_matches(const struct arm64_cpu_capabilities *entry,
>  	return false;
>  }
>  
> +extern cpumask_t aarch32_el0_mask;
> +
>  extern DECLARE_BITMAP(cpu_hwcaps, ARM64_NCAPS);
>  extern struct static_key_false cpu_hwcap_keys[ARM64_NCAPS];
>  extern struct static_key_false arm64_const_caps_ready;
> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> index 0f7307c8ad80..662bbc2b15cd 100644
> --- a/arch/arm64/kernel/cpufeature.c
> +++ b/arch/arm64/kernel/cpufeature.c
> @@ -1723,6 +1723,13 @@ cpucap_panic_on_conflict(const struct arm64_cpu_capabilities *cap)
>  	return !!(cap->type & ARM64_CPUCAP_PANIC_ON_CONFLICT);
>  }
>  
> +cpumask_t aarch32_el0_mask;
> +static void cpu_enable_aarch32_el0(struct arm64_cpu_capabilities const *cap)
> +{
> +	if (has_cpuid_feature(cap, SCOPE_LOCAL_CPU))
> +		cpumask_set_cpu(smp_processor_id(), &aarch32_el0_mask);
> +}
> +
>  static const struct arm64_cpu_capabilities arm64_features[] = {
>  	{
>  		.desc = "GIC system register CPU interface",
> @@ -1809,6 +1816,7 @@ static const struct arm64_cpu_capabilities arm64_features[] = {
>  	{
>  		.capability = ARM64_HAS_ASYM_32BIT_EL0,
>  		.type = ARM64_CPUCAP_WEAK_LOCAL_CPU_FEATURE,
> +		.cpu_enable = cpu_enable_aarch32_el0,
>  		.matches = has_cpuid_feature,
>  		.sys_reg = SYS_ID_AA64PFR0_EL1,
>  		.sign = FTR_UNSIGNED,
> diff --git a/drivers/base/cpu.c b/drivers/base/cpu.c
> index d2136ab9b14a..569baacde508 100644
> --- a/drivers/base/cpu.c
> +++ b/drivers/base/cpu.c
> @@ -459,6 +459,15 @@ EXPORT_SYMBOL_GPL(cpu_device_create);
>  static DEVICE_ATTR(modalias, 0444, print_cpu_modalias, NULL);
>  #endif
>  
> +#ifdef CONFIG_ARM64
> +static ssize_t print_aarch32_online(struct device *dev,
> +				       struct device_attribute *attr, char *buf)
> +{
> +	return cpumap_print_to_pagebuf(true, buf, &aarch32_el0_mask);
> +}
> +static DEVICE_ATTR(aarch32_online, 0444, print_aarch32_online, NULL);

DEVICE_ATTR_RO()?

> +#endif

Hah, no, no arch-specific stuff in here, sorry.  Please do this properly
in your arch-specific code only.

thanks,

greg k-h