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From: Mark Rutland <mark.rutland@arm.com>
To: Robin Murphy <robin.murphy@arm.com>
Cc: Dave Martin <Dave.Martin@arm.com>,
	marc.zyngier@arm.com, catalin.marinas@arm.com,
	will.deacon@arm.com, linux-kernel@vger.kernel.org,
	linux@dominikbrodowski.net, james.morse@arm.com,
	viro@zeniv.linux.org.uk, linux-fsdevel@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH 02/18] arm64: move SCTLR_EL{1,2} assertions to <asm/sysreg.h>
Date: Mon, 14 May 2018 13:06:10 +0100	[thread overview]
Message-ID: <20180514120609.3vd2s4l3rknd2b3e@lakrids.cambridge.arm.com> (raw)
In-Reply-To: <8d66d231-d8a2-07c1-6c5b-24c9474c851f@arm.com>

On Mon, May 14, 2018 at 12:56:09PM +0100, Robin Murphy wrote:
> On 14/05/18 12:20, Dave Martin wrote:
> > On Mon, May 14, 2018 at 11:08:59AM +0100, Mark Rutland wrote:
> > > On Mon, May 14, 2018 at 11:00:53AM +0100, Dave Martin wrote:
> > > > On Mon, May 14, 2018 at 10:46:24AM +0100, Mark Rutland wrote:
> > > > > -/* Check all the bits are accounted for */
> > > > > -#define SCTLR_EL2_BUILD_BUG_ON_MISSING_BITS	BUILD_BUG_ON((SCTLR_EL2_SET ^ SCTLR_EL2_CLEAR) != ~0)
> > > > > -
> > > > > +#if (SCTLR_EL2_SET ^ SCTLR_EL2_CLEAR) != 0xffffffff
> > > > > +#error "Inconsistent SCTLR_EL2 set/clear bits"
> > > > > +#endif
> > > > 
> > > > Can we have a comment on the != 0xffffffff versus != ~0 here?
> > > > 
> > > > The subtle differences in evaluation semantics between #if and
> > > > other contexts here may well trip people up during maintenance...
> > > 
> > > Do you have any suggestion as to the wording?
> > > 
> > > I'm happy to add a comment, but I don't really know what to say.
> > 
> > 
> > How about the following?
> > 
> > /* Watch out for #if evaluation rules: ~0 is not ~(int)0! */
> 
> Or, more formally, perhaps something even less vague like "Note that in
> preprocessor arithmetic these constants are effectively of type intmax_t,
> which is 64-bit, thus ~0 is not what we want."

I'll drop something in the commit message to that effect, rather than a
comment.

A comment will end up terse and vague or large and bloatsome (and
redundant as we have this pattern twice).

Anyone attempting to "clean" this up will find things break, and they can
look at the git log to find out why it is the way it is...

Thanks,
Mark.

  reply	other threads:[~2018-05-14 12:06 UTC|newest]

Thread overview: 63+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-05-14  9:46 [PATCH 00/18] arm64: invoke syscalls with pt_regs Mark Rutland
2018-05-14  9:46 ` [PATCH 01/18] arm64: consistently use unsigned long for thread flags Mark Rutland
2018-05-14  9:57   ` Dave Martin
2018-05-14  9:46 ` [PATCH 02/18] arm64: move SCTLR_EL{1,2} assertions to <asm/sysreg.h> Mark Rutland
2018-05-14 10:00   ` Dave Martin
2018-05-14 10:08     ` Mark Rutland
2018-05-14 11:20       ` Dave Martin
2018-05-14 11:56         ` Robin Murphy
2018-05-14 12:06           ` Mark Rutland [this message]
2018-05-14 12:41             ` Dave Martin
2018-05-14 13:10               ` Mark Rutland
2018-05-14  9:46 ` [PATCH 03/18] arm64: introduce sysreg_clear_set() Mark Rutland
2018-05-14 10:04   ` Dave Martin
2018-05-14  9:46 ` [PATCH 04/18] arm64: kill config_sctlr_el1() Mark Rutland
2018-05-14 10:05   ` Dave Martin
2018-05-14  9:46 ` [PATCH 05/18] arm64: kill change_cpacr() Mark Rutland
2018-05-14 10:06   ` Dave Martin
2018-05-14  9:46 ` [PATCH 06/18] arm64: move sve_user_{enable,disable} to <asm/fpsimd.h> Mark Rutland
2018-05-14 11:06   ` [PATCH 06/18] arm64: move sve_user_{enable, disable} " Dave Martin
2018-05-15 10:39     ` Mark Rutland
2018-05-15 12:19       ` Dave Martin
2018-05-15 16:33         ` Mark Rutland
2018-05-16  9:01           ` Dave Martin
2018-06-01 10:29             ` Mark Rutland
2018-06-01 10:42               ` Dave Martin
2018-05-14  9:46 ` [PATCH 07/18] arm64: remove sigreturn wrappers Mark Rutland
2018-05-14 11:07   ` Dave Martin
2018-05-14  9:46 ` [PATCH 08/18] arm64: convert raw syscall invocation to C Mark Rutland
2018-05-14 11:07   ` Dave Martin
2018-05-14 11:41     ` Mark Rutland
2018-05-14 12:53       ` Dave Martin
2018-05-14 20:24       ` Dominik Brodowski
2018-05-15  8:22         ` Mark Rutland
2018-05-15 10:01           ` Dominik Brodowski
2018-05-15 10:13             ` Mark Rutland
2018-05-14 18:00   ` Dominik Brodowski
2018-05-15  8:18     ` Mark Rutland
2018-05-14  9:46 ` [PATCH 09/18] arm64: convert syscall trace logic " Mark Rutland
2018-05-14  9:46 ` [PATCH 10/18] arm64: convert native/compat syscall entry " Mark Rutland
2018-05-14 11:07   ` Dave Martin
2018-05-14 11:58     ` Mark Rutland
2018-05-14 14:43       ` Dave Martin
2018-05-14 15:01         ` Mark Rutland
2018-05-14  9:46 ` [PATCH 11/18] arm64: zero GPRs upon entry from EL0 Mark Rutland
2018-05-14 11:07   ` Dave Martin
2018-05-14  9:46 ` [PATCH 12/18] kernel: add ksys_personality() Mark Rutland
2018-05-14 11:08   ` Dave Martin
2018-05-14 12:07   ` Christoph Hellwig
2018-05-15  9:56     ` Mark Rutland
2018-05-14  9:46 ` [PATCH 13/18] kernel: add kcompat_sys_{f,}statfs64() Mark Rutland
2018-05-14 17:14   ` Mark Rutland
2018-05-14 20:34     ` Dominik Brodowski
2018-05-15  9:53       ` Mark Rutland
2018-05-15  9:58         ` Dominik Brodowski
2018-05-14  9:46 ` [PATCH 14/18] arm64: remove in-kernel call to sys_personality() Mark Rutland
2018-05-14  9:46 ` [PATCH 15/18] arm64: use {COMPAT,}SYSCALL_DEFINE0 for sigreturn Mark Rutland
2018-05-14  9:46 ` [PATCH 16/18] arm64: use SYSCALL_DEFINE6() for mmap Mark Rutland
2018-05-14  9:46 ` [PATCH 17/18] arm64: convert compat wrappers to C Mark Rutland
2018-05-14 12:10   ` Christoph Hellwig
2018-05-14 12:43     ` Mark Rutland
2018-05-14  9:46 ` [PATCH 18/18] arm64: implement syscall wrappers Mark Rutland
2018-05-14 20:57   ` Dominik Brodowski
2018-05-15  8:37     ` Mark Rutland

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