On Wed, 2024-03-06 at 16:31 +0100, Jan Beulich wrote:
> On 26.02.2024 18:38, Oleksii Kurochko wrote:
> > --- /dev/null
> > +++ b/xen/arch/riscv/include/asm/atomic.h
> > @@ -0,0 +1,296 @@
> > +/* 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>
> > +
> > +#include <asm-generic/atomic-ops.h>
>
> While, because of the forward decls in xen/atomic.h, having this
> #include
> works, I wonder if it wouldn't better be placed further down. The
> compiler
> will likely have an easier time when it sees the inline definitions
> ahead
> of any uses.
Do you mean to move it after #define __atomic_release_fence() ?
>
> > +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<X>_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;
> > + case 8: *(uint32_t *)res = readq(p); break;
>
> This is the point where the lack of constraints in io.h (see my
> respective
> comment) becomes actually harmful: You're accessing not MMIO, but
> compiler-
> visible variables here. It needs to know which ones are read ...
>
> > + 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; \
> > +})
> > +
> > +#define write_atomic(p, x) \
> > +({ \
> > + typeof(*p) x__ = (x); \
> > + switch ( sizeof(*p) ) \
> > + { \
> > + case 1: writeb(x__, p); break; \
> > + case 2: writew(x__, p); break; \
> > + case 4: writel(x__, p); break; \
> > + case 8: writeq(x__, p); break; \
>
> ... or written.
>
> Nit: There's a stray blank in the writeb() invocation.
>
> > + default: __bad_atomic_size(); break; \
> > + } \
> > + x__; \
> > +})
> > +
> > +#define add_sized(p, x) \
> > +({ \
> > + typeof(*(p)) x__ = (x); \
> > + switch ( sizeof(*(p)) ) \
>
> Like you have it here, {read,write}_atomic() also need p properly
> parenthesized. There look to be more parenthesization issues further
> down.
>
> > + { \
> > + case 1: writeb(read_atomic(p) + x__, p); break; \
> > + case 2: writew(read_atomic(p) + x__, p); break; \
> > + case 4: writel(read_atomic(p) + x__, p); break; \
> > + default: __bad_atomic_size(); break; \
> > + } \
> > +})
>
> Any reason this doesn't have an 8-byte case? x86'es at least has one.
Just missed to add and no compiler error I had, but I'll added case 8.
>
> > +#define __atomic_acquire_fence() \
> > + __asm__ __volatile__ ( RISCV_ACQUIRE_BARRIER "" ::: "memory" )
> > +
> > +#define __atomic_release_fence() \
> > + __asm__ __volatile__ ( RISCV_RELEASE_BARRIER "" ::: "memory" )
>
> Elsewhere you use asm volatile() - why __asm__ __volatile__() here?
> Or why not there (cmpxchg.h, io.h)?
It is how it was defined in Linux kernel, so I decided to use their
code style, but considering this macros likely not to be changed I can
update this lines with asm volatile.
>
> > +/*
> > + * First, the atomic ops that have no ordering constraints and
> > therefor don't
> > + * have the AQ or RL bits set. These don't return anything, so
> > there's only
> > + * one version to worry about.
> > + */
> > +#define ATOMIC_OP(op, asm_op, I, asm_type, c_type, prefix) \
> > +static inline \
> > +void atomic##prefix##_##op(c_type i, atomic##prefix##_t *v) \
> > +{ \
> > + __asm__ __volatile__ ( \
> > + " amo" #asm_op "." #asm_type " zero, %1, %0" \
> > + : "+A" (v->counter) \
> > + : "r" (I) \
> > + : "memory" ); \
> > +} \
> > +
> > +#define ATOMIC_OPS(op, asm_op, I) \
> > + ATOMIC_OP (op, asm_op, I, w, int, )
> > +
> > +ATOMIC_OPS(add, add, i)
> > +ATOMIC_OPS(sub, add, -i)
> > +ATOMIC_OPS(and, and, i)
> > +ATOMIC_OPS( or, or, i)
> > +ATOMIC_OPS(xor, xor, i)
> > +
> > +#undef ATOMIC_OP
> > +#undef ATOMIC_OPS
> > +
> > +/*
> > + * Atomic ops that have ordered, relaxed, acquire, and release
> > variants.
> > + * There's two flavors of these: the arithmatic ops have both
> > fetch and return
> > + * versions, while the logical ops only have fetch versions.
> > + */
> > +#define ATOMIC_FETCH_OP(op, asm_op, I, asm_type, c_type,
> > prefix) \
> > +static
> > inline \
> > +c_type atomic##prefix##_fetch_##op##_relaxed(c_type
> > i, \
> > + atomic##prefix##_t
> > *v) \
> > +{
> > \
> > + register c_type
> > ret; \
> > + __asm__ __volatile__
> > ( \
> > + " amo" #asm_op "." #asm_type " %1, %2,
> > %0" \
> > + : "+A" (v->counter), "=r"
> > (ret) \
> > + : "r"
> > (I) \
> > + : "memory"
> > ); \
> > + return
> > ret; \
> > +}
> > \
> > +static
> > inline \
> > +c_type atomic##prefix##_fetch_##op(c_type i, atomic##prefix##_t
> > *v) \
> > +{
> > \
> > + register c_type
> > ret; \
> > + __asm__ __volatile__
> > ( \
> > + " amo" #asm_op "." #asm_type ".aqrl %1, %2,
> > %0" \
> > + : "+A" (v->counter), "=r"
> > (ret) \
> > + : "r"
> > (I) \
> > + : "memory"
> > ); \
> > + return
> > ret; \
> > +}
> > +
> > +#define ATOMIC_OP_RETURN(op, asm_op, c_op, I, asm_type, c_type,
> > prefix) \
> > +static
> > inline \
> > +c_type atomic##prefix##_##op##_return_relaxed(c_type
> > i, \
> > + atomic##prefix##_t
> > *v) \
> > +{
> > \
> > + return atomic##prefix##_fetch_##op##_relaxed(i, v) c_op
> > I; \
> > +}
> > \
> > +static
> > inline \
> > +c_type atomic##prefix##_##op##_return(c_type i, atomic##prefix##_t
> > *v) \
> > +{
> > \
> > + return atomic##prefix##_fetch_##op(i, v) c_op
> > I; \
> > +}
> > +
> > +#define ATOMIC_OPS(op, asm_op, c_op,
> > I) \
> > + ATOMIC_FETCH_OP( op, asm_op, I, w, int,
> > ) \
> > + ATOMIC_OP_RETURN(op, asm_op, c_op, I, w, int, )
>
> What purpose is the last macro argument when you only ever pass
> nothing
> for it (here and ...
>
> > +ATOMIC_OPS(add, add, +, i)
> > +ATOMIC_OPS(sub, add, +, -i)
> > +
> > +#undef ATOMIC_OPS
> > +
> > +#define ATOMIC_OPS(op, asm_op, I) \
> > + ATOMIC_FETCH_OP(op, asm_op, I, w, int, )
>
> ... here)?
for generic ATOMIC64 it is not used:
#ifdef CONFIG_GENERIC_ATOMIC64
#define ATOMIC_OPS(op, asm_op, c_op,
I) \
ATOMIC_FETCH_OP( op, asm_op, I, w, int,
) \
ATOMIC_OP_RETURN(op, asm_op, c_op, I, w, int, )
#else
#define ATOMIC_OPS(op, asm_op, c_op,
I) \
ATOMIC_FETCH_OP( op, asm_op, I, w, int,
) \
ATOMIC_OP_RETURN(op, asm_op, c_op, I, w, int,
) \
ATOMIC_FETCH_OP( op, asm_op, I, d, s64,
64) \
ATOMIC_OP_RETURN(op, asm_op, c_op, I, d, s64, 64)
#endif
( the code is from Linux kernel )
Only CONFIG_GENERIC_ATOMIC64=y was ported to Xen.
>
> > +ATOMIC_OPS(and, and, i)
> > +ATOMIC_OPS( or, or, i)
> > +ATOMIC_OPS(xor, xor, i)
> > +
> > +#undef ATOMIC_OPS
> > +
> > +#undef ATOMIC_FETCH_OP
> > +#undef ATOMIC_OP_RETURN
> > +
> > +/* This is required to provide a full barrier on success. */
> > +static inline int atomic_add_unless(atomic_t *v, int a, int u)
> > +{
> > + int prev, rc;
> > +
> > + __asm__ __volatile__ (
> > + "0: lr.w %[p], %[c]\n"
> > + " beq %[p], %[u], 1f\n"
> > + " add %[rc], %[p], %[a]\n"
> > + " sc.w.rl %[rc], %[rc], %[c]\n"
> > + " bnez %[rc], 0b\n"
> > + RISCV_FULL_BARRIER
> > + "1:\n"
> > + : [p] "=&r" (prev), [rc] "=&r" (rc), [c] "+A" (v->counter)
> > + : [a] "r" (a), [u] "r" (u)
> > + : "memory");
> > + return prev;
> > +}
> > +
> > +/*
> > + * atomic_{cmp,}xchg is required to have exactly the same ordering
> > semantics as
> > + * {cmp,}xchg and the operations that return, so they need a full
> > barrier.
> > + */
> > +#define ATOMIC_OP(c_t, prefix, size) \
> > +static inline \
> > +c_t atomic##prefix##_xchg_relaxed(atomic##prefix##_t *v, c_t n) \
> > +{ \
> > + return __xchg_generic(&(v->counter), n, size, "", "", ""); \
>
> The inner parentheses aren't really needed here, are they?
>
> > +} \
> > +static inline \
> > +c_t atomic##prefix##_xchg_acquire(atomic##prefix##_t *v, c_t n) \
> > +{ \
> > + return __xchg_generic(&(v->counter), n, size, \
> > + "", "", RISCV_ACQUIRE_BARRIER); \
> > +} \
> > +static inline \
> > +c_t atomic##prefix##_xchg_release(atomic##prefix##_t *v, c_t n) \
> > +{ \
> > + return __xchg_generic(&(v->counter), n, size, \
> > + "", RISCV_RELEASE_BARRIER, ""); \
> > +} \
> > +static inline \
> > +c_t atomic##prefix##_xchg(atomic##prefix##_t *v, c_t n) \
> > +{ \
> > + return __xchg_generic(&(v->counter), n, size, \
> > + ".aqrl", "", ""); \
> > +} \
> > +static inline \
> > +c_t atomic##prefix##_cmpxchg_relaxed(atomic##prefix##_t *v, \
> > + c_t o, c_t n) \
> > +{ \
> > + return __cmpxchg_generic(&(v->counter), o, n, size, \
> > + "", "", ""); \
> > +} \
> > +static inline \
> > +c_t atomic##prefix##_cmpxchg_acquire(atomic##prefix##_t *v, \
> > + c_t o, c_t n) \
> > +{ \
> > + return __cmpxchg_generic(&(v->counter), o, n, size, \
> > + "", "", RISCV_ACQUIRE_BARRIER); \
> > +} \
> > +static inline \
> > +c_t atomic##prefix##_cmpxchg_release(atomic##prefix##_t *v, \
> > + c_t o, c_t n) \
> > +{
> > \
>
> A hard tab looks to have been left here.
>
> > + return __cmpxchg_generic(&(v->counter), o, n, size, \
> > + "", RISCV_RELEASE_BARRIER, ""); \
> > +} \
> > +static inline \
> > +c_t atomic##prefix##_cmpxchg(atomic##prefix##_t *v, c_t o, c_t n)
> > \
> > +{ \
> > + return __cmpxchg_generic(&(v->counter), o, n, size, \
> > + ".rl", "", " fence rw, rw\n"); \
> > +}
> > +
> > +#define ATOMIC_OPS() \
> > + ATOMIC_OP(int, , 4)
> > +
> > +ATOMIC_OPS()
> > +
> > +#undef ATOMIC_OPS
> > +#undef ATOMIC_OP
> > +
> > +static inline int atomic_sub_if_positive(atomic_t *v, int offset)
> > +{
> > + int prev, rc;
> > +
> > + __asm__ __volatile__ (
> > + "0: lr.w %[p], %[c]\n"
> > + " sub %[rc], %[p], %[o]\n"
> > + " bltz %[rc], 1f\n"
> > + " sc.w.rl %[rc], %[rc], %[c]\n"
> > + " bnez %[rc], 0b\n"
> > + " fence rw, rw\n"
> > + "1:\n"
> > + : [p] "=&r" (prev), [rc] "=&r" (rc), [c] "+A" (v->counter)
> > + : [o] "r" (offset)
> > + : "memory" );
> > + return prev - offset;
> > +}
> > +
> > +#define atomic_dec_if_positive(v) atomic_sub_if_positive(v,
> > 1)
>
> Hmm, PPC for some reason also has the latter, but for both: Are they
> indeed
> going to be needed in RISC-V code? They certainly look unnecessary
> for the
> purpose of this series (allowing common code to build).
I checked my private branched and I don't use it still, so it makes
sense to drop it.
>
> > --- /dev/null
> > +++ b/xen/include/asm-generic/atomic-ops.h
> > @@ -0,0 +1,92 @@
> > +#/* SPDX-License-Identifier: GPL-2.0 */
> > +#ifndef _ASM_GENERIC_ATOMIC_OPS_H_
> > +#define _ASM_GENERIC_ATOMIC_OPS_H_
> > +
> > +#include <xen/atomic.h>
> > +#include <xen/lib.h>
>
> If I'm not mistaken this header provides default implementations for
> every
> xen/atomic.h-provided forward inline declaration that can be
> synthesized
> from other atomic functions. I think a comment to this effect would
> want
> adding somewhere here.
I think we can drop this inclusion here as inclusion of asm-
generic/atomic-ops.h will be always go with inclusion of xen/atomic.h.
~ Oleksii
