linux-kernel - Re: [PATCH 1/2] s390/rwonce: add READ_ONCE_ALIGNED

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Message-ID: <Y/o8cmyc7IW+28S2@hirez.programming.kicks-ass.net>
Date:   Sat, 25 Feb 2023 17:50:58 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Heiko Carstens <hca@...ux.ibm.com>
Cc:     Vasily Gorbik <gor@...ux.ibm.com>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        Thomas Richter <tmricht@...ux.ibm.com>,
        Sven Schnelle <svens@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] s390/rwonce: add READ_ONCE_ALIGNED_128() macro

On Fri, Feb 24, 2023 at 11:02:36AM +0100, Heiko Carstens wrote:
> Add an s390 specific READ_ONCE_ALIGNED_128() helper, which can be used for
> fast block concurrent (atomic) 128-bit accesses.
> 
> The used lpq instruction requires 128-bit alignment. This is also the
> reason why the compiler doesn't emit this instruction if __READ_ONCE() is
> used for 128-bit accesses.

Does your u128 not have natural alignment? Does it help if you force
align the u128 type?

> 
> Signed-off-by: Heiko Carstens <hca@...ux.ibm.com>
> ---
>  arch/s390/include/asm/rwonce.h | 31 +++++++++++++++++++++++++++++++
>  1 file changed, 31 insertions(+)
>  create mode 100644 arch/s390/include/asm/rwonce.h
> 
> diff --git a/arch/s390/include/asm/rwonce.h b/arch/s390/include/asm/rwonce.h
> new file mode 100644
> index 000000000000..91fc24520e82
> --- /dev/null
> +++ b/arch/s390/include/asm/rwonce.h
> @@ -0,0 +1,31 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +
> +#ifndef __ASM_S390_RWONCE_H
> +#define __ASM_S390_RWONCE_H
> +
> +#include <linux/compiler_types.h>
> +
> +/*
> + * Use READ_ONCE_ALIGNED_128() for 128-bit block concurrent (atomic) read
> + * accesses. Note that x must be 128-bit aligned, otherwise a specification
> + * exception is generated.
> + */
> +#define READ_ONCE_ALIGNED_128(x)			\
> +({							\
> +	union {						\
> +		typeof(x) __x;				\
> +		__uint128_t val;			\
> +	} __u;						\
> +							\
> +	BUILD_BUG_ON(sizeof(x) != 16);			\
> +	asm volatile(					\
> +		"	lpq	%[val],%[_x]\n"		\
> +		: [val] "=d" (__u.val)			\
> +		: [_x] "QS" (x)				\
> +		: "memory");				\
> +	__u.__x;					\
> +})
> +
> +#include <asm-generic/rwonce.h>
> +
> +#endif	/* __ASM_S390_RWONCE_H */
> -- 
> 2.37.2
>