Re: [PATCH v8 08/17] xen/riscv: introduce atomic.h

[Jan Beulich <jbeulich@xxxxxxxx>]

On 17.04.2024 12:04, Oleksii Kurochko wrote:
> --- /dev/null
> +++ b/xen/arch/riscv/include/asm/atomic.h
> @@ -0,0 +1,281 @@
> +/* SPDX-License-Identifier: GPL-2.0-only */
> +/*
> + * Taken and modified from Linux.
> + *
> + * The following changes were done:
> + * - * atomic##prefix##_*xchg_*(atomic##prefix##_t *v, c_t n) were updated
> + *     to use__*xchg_generic()
> + * - drop casts in write_atomic() as they are unnecessary
> + * - drop introduction of WRITE_ONCE() and READ_ONCE().
> + *   Xen provides ACCESS_ONCE()
> + * - remove zero-length array access in read_atomic()
> + * - drop defines similar to pattern
> + *   #define atomic_add_return_relaxed   atomic_add_return_relaxed
> + * - move not RISC-V specific functions to asm-generic/atomics-ops.h
> + * 
> + * Copyright (C) 2007 Red Hat, Inc. All Rights Reserved.
> + * Copyright (C) 2012 Regents of the University of California
> + * Copyright (C) 2017 SiFive
> + * Copyright (C) 2024 Vates SAS
> + */
> +
> +#ifndef _ASM_RISCV_ATOMIC_H
> +#define _ASM_RISCV_ATOMIC_H
> +
> +#include <xen/atomic.h>
> +
> +#include <asm/cmpxchg.h>
> +#include <asm/fence.h>
> +#include <asm/io.h>
> +#include <asm/system.h>
> +
> +void __bad_atomic_size(void);
> +
> +/*
> + * Legacy from Linux kernel. For some reason they wanted to have ordered
> + * read/write access. Thereby read* is used instead of read*_cpu()
> + */
> +static always_inline void read_atomic_size(const volatile void *p,
> +                                           void *res,
> +                                           unsigned int size)
> +{
> +    switch ( size )
> +    {
> +    case 1: *(uint8_t *)res = readb(p); break;
> +    case 2: *(uint16_t *)res = readw(p); break;
> +    case 4: *(uint32_t *)res = readl(p); break;
> +#ifndef CONFIG_RISCV_32
> +    case 8: *(uint32_t *)res = readq(p); break;
> +#endif
> +    default: __bad_atomic_size(); break;
> +    }
> +}
> +
> +#define read_atomic(p) ({                                   \
> +    union { typeof(*(p)) val; char c[sizeof(*(p))]; } x_;   \
> +    read_atomic_size(p, x_.c, sizeof(*(p)));                \
> +    x_.val;                                                 \
> +})
> +
> +static always_inline void _write_atomic(volatile void *p,
> +                                       unsigned long x, unsigned int size)
> +{
> +    switch ( size )
> +    {
> +    case 1: writeb(x, p); break;
> +    case 2: writew(x, p); break;
> +    case 4: writel(x, p); break;
> +#ifndef CONFIG_RISCV_32
> +    case 8: writeq(x, p); break;
> +#endif
> +    default: __bad_atomic_size(); break;
> +    }
> +}
> +
> +#define write_atomic(p, x)                              \
> +({                                                      \
> +    typeof(*(p)) x_ = (x);                              \
> +    _write_atomic((p), x_, sizeof(*(p)));               \

Nit: There are still excess parentheses here.

> +    x_;                                                 \

Why is this? The macro isn't supposed to "return" a value, is it?

> +})
> +
> +static always_inline void _add_sized(volatile void *p,
> +                                     unsigned long x, unsigned int size)
> +{
> +    switch ( size )
> +    {
> +    case 1:
> +    {
> +        volatile uint8_t *ptr = (volatile uint8_t *)p;

Here and below: Why the casts?

Jan