linux-kernel - Re: [PATCH V2] arm64/cpuinfo: Move HWCAP name arrays alongside their bit definitions

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Date:   Thu, 14 May 2020 08:36:13 +0100
From:   Will Deacon <will@...nel.org>
To:     Anshuman Khandual <anshuman.khandual@....com>
Cc:     Dave Martin <Dave.Martin@....com>,
        linux-arm-kernel@...ts.infradead.org,
        Mark Rutland <mark.rutland@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        linux-kernel@...r.kernel.org, Mark Brown <broonie@...nel.org>,
        Ard Biesheuvel <ardb@...nel.org>
Subject: Re: [PATCH V2] arm64/cpuinfo: Move HWCAP name arrays alongside their
 bit definitions

On Thu, May 14, 2020 at 07:14:58AM +0530, Anshuman Khandual wrote:
> On 05/13/2020 08:34 PM, Dave Martin wrote:
> > On Thu, May 07, 2020 at 06:59:10PM +0530, Anshuman Khandual wrote:
> >> diff --git a/arch/arm64/include/asm/hwcap.h b/arch/arm64/include/asm/hwcap.h
> >> index 0f00265248b5..589ac02e1ddd 100644
> >> --- a/arch/arm64/include/asm/hwcap.h
> >> +++ b/arch/arm64/include/asm/hwcap.h
> >> @@ -8,18 +8,27 @@
> >>  #include <uapi/asm/hwcap.h>
> >>  #include <asm/cpufeature.h>
> >>  
> >> +#define COMPAT_HWCAP_SWP	(1 << 0)
> >>  #define COMPAT_HWCAP_HALF	(1 << 1)
> >>  #define COMPAT_HWCAP_THUMB	(1 << 2)
> >> +#define COMPAT_HWCAP_26BIT	(1 << 3)
> >>  #define COMPAT_HWCAP_FAST_MULT	(1 << 4)
> >> +#define COMPAT_HWCAP_FPA	(1 << 5)
> >>  #define COMPAT_HWCAP_VFP	(1 << 6)
> >>  #define COMPAT_HWCAP_EDSP	(1 << 7)
> >> +#define COMPAT_HWCAP_JAVA	(1 << 8)
> >> +#define COMPAT_HWCAP_IWMMXT	(1 << 9)
> >> +#define COMPAT_HWCAP_CRUNCH	(1 << 10)
> >> +#define COMPAT_HWCAP_THUMBEE	(1 << 11)
> >>  #define COMPAT_HWCAP_NEON	(1 << 12)
> >>  #define COMPAT_HWCAP_VFPv3	(1 << 13)
> >> +#define COMPAT_HWCAP_VFPV3D16	(1 << 14)
> >>  #define COMPAT_HWCAP_TLS	(1 << 15)
> >>  #define COMPAT_HWCAP_VFPv4	(1 << 16)
> >>  #define COMPAT_HWCAP_IDIVA	(1 << 17)
> >>  #define COMPAT_HWCAP_IDIVT	(1 << 18)
> >>  #define COMPAT_HWCAP_IDIV	(COMPAT_HWCAP_IDIVA|COMPAT_HWCAP_IDIVT)
> >> +#define COMPAT_HWCAP_VFPD32	(1 << 19)
> >>  #define COMPAT_HWCAP_LPAE	(1 << 20)
> >>  #define COMPAT_HWCAP_EVTSTRM	(1 << 21)
> > 
> > With the possible exception of SWP (does the swp emulation allow us to
> > report this as supported?), I think all these weren't mentioned because
> > they aren't included in ARMv8 and so can never be reported.
> > 
> > If we find ourselves reporting them, there's a bug somewhere.
> > 
> > So, can we just default all obsolete string entries to NULL?
> > 
> > When generating the cpuinfo strings we could WARN and just emit an empty
> > string for that hwcap.
> 
> All these above will be a change in the existing user visible behavior on
> the system and this patch never intended to create one.

Why is it a change? We've never reported e.g. "java" on an arm64 kernel, so
I agree with Dave that we shouldn't be adding this string. If it /ever/ ends
up in userspace it's because something has gone horribly wrong. NULL would
be much better. Couldn't you achieve that by simply omitting these entries?
e.g. deleting things like:

	[COMPAT_KERNEL_HWCAP(JAVA)]     = "java",

completely (including the COMPAT_HWCAP_JAVA definition)?

> Hence, I will just defer this to maintainers on whether we should change
> existing /proc/cpuinfo output (including non-practically-possible ones on
> ARMv8) or even treat swap emulation as SWP.

SWP is fine because we emulate it and so userspace can use it. Removing that
*would* be a change in behaviour. I don't think the compat ABI is broken
here, so please don't change it without good reason.

Will