Re: [PATCH v3 05/11] percpu: Wire up cmpxchg128

Peter Zijlstra
2023-05-25T12:51:08Z

On Mon, May 15, 2023 at 09:57:04AM +0200, Peter Zijlstra wrote:
> In order to replace cmpxchg_double() with the newly minted
> cmpxchg128() family of functions, wire it up in this_cpu_cmpxchg().
>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>

> ---
> --- a/arch/x86/include/asm/percpu.h
> +++ b/arch/x86/include/asm/percpu.h
> @@ -210,6 +210,65 @@ do { \
> (typeof(_var))(unsigned long) pco_old__; \
> })
>
> +#if defined(CONFIG_X86_32) && defined(CONFIG_X86_CMPXCHG64)
> +#define percpu_cmpxchg64_op(size, qual, _var, _oval, _nval) \
> +({ \
> + union { \
> + u64 var; \
> + struct { \
> + u32 low, high; \
> + }; \
> + } old__, new__; \
> + \
> + old__.var = _oval; \
> + new__.var = _nval; \
> + \
> + asm qual ("cmpxchg8b " __percpu_arg([var]) \
> + : [var] "+m" (_var), \
> + "+a" (old__.low), \
> + "+d" (old__.high) \
> + : "b" (new__.low), \
> + "c" (new__.high) \
> + : "memory"); \
> + \
> + old__.var; \
> +})
> +
> +#define raw_cpu_cmpxchg64(pcp, oval, nval) percpu_cmpxchg64_op(8, , pcp, oval, nval)
> +#define this_cpu_cmpxchg64(pcp, oval, nval) percpu_cmpxchg64_op(8, volatile, pcp, oval, nval)
> +#endif
> +
> +#ifdef CONFIG_X86_64
> +#define raw_cpu_cmpxchg64(pcp, oval, nval) percpu_cmpxchg_op(8, , pcp, oval, nval);
> +#define this_cpu_cmpxchg64(pcp, oval, nval) percpu_cmpxchg_op(8, volatile, pcp, oval, nval);
> +
> +#define percpu_cmpxchg128_op(size, qual, _var, _oval, _nval) \
> +({ \
> + union { \
> + u128 var; \
> + struct { \
> + u64 low, high; \
> + }; \
> + } old__, new__; \
> + \
> + old__.var = _oval; \
> + new__.var = _nval; \
> + \
> + asm qual ("cmpxchg16b " __percpu_arg([var]) \
> + : [var] "+m" (_var), \
> + "+a" (old__.low), \
> + "+d" (old__.high) \
> + : "b" (new__.low), \
> + "c" (new__.high) \
> + : "memory"); \
> + \
> + old__.var; \
> +})
> +
> +#define raw_cpu_cmpxchg128(pcp, oval, nval) percpu_cmpxchg128_op(16, , pcp, oval, nval)
> +#define this_cpu_cmpxchg128(pcp, oval, nval) percpu_cmpxchg128_op(16, volatile, pcp, oval, nval)
> +#endif
> +
> /*
> * this_cpu_read() makes gcc load the percpu variable every time it is
> * accessed while this_cpu_read_stable() allows the value to be cached.

Since this_cpu_cmpxchg*() is assumed to always be present (their usage
is not guarded with system_has_cmpxchg*() it needs a fallback for then
the instruction is not present.

The below has been tested with clearcpuid=cx16; adding an obvious defect
in the fallback implemention crashes the kernel, with the code as
presented it boots.

Build tested i386-defconfig.

(the things we do for museum pieces :/)

---
include/asm/percpu.h | 25 +++++++++++---------
lib/Makefile | 3 +-
lib/cmpxchg16b_emu.S | 43 ++++++++++++++++++++--------------
lib/cmpxchg8b_emu.S | 63 +++++++++++++++++++++++++++++++++++++++------------
4 files changed, 90 insertions(+), 44 deletions(-)

--- a/arch/x86/include/asm/percpu.h
+++ b/arch/x86/include/asm/percpu.h
@@ -210,7 +210,7 @@ do { \
(typeof(_var))(unsigned long) pco_old__; \
})

-#if defined(CONFIG_X86_32) && defined(CONFIG_X86_CMPXCHG64)
+#ifdef CONFIG_X86_32
#define percpu_cmpxchg64_op(size, qual, _var, _oval, _nval) \
({ \
union { \
@@ -223,13 +223,14 @@ do { \
old__.var = _oval; \
new__.var = _nval; \
\
- asm qual ("cmpxchg8b " __percpu_arg([var]) \
+ asm qual (ALTERNATIVE("leal %P[var], %%esi; call this_cpu_cmpxchg8b_emu", \
+ "cmpxchg8b " __percpu_arg([var]), X86_FEATURE_CX8) \
: [var] "+m" (_var), \
"+a" (old__.low), \
"+d" (old__.high) \
: "b" (new__.low), \
"c" (new__.high) \
- : "memory"); \
+ : "memory", "esi"); \
\
old__.var; \
})
@@ -254,13 +255,14 @@ do { \
old__.var = _oval; \
new__.var = _nval; \
\
- asm qual ("cmpxchg16b " __percpu_arg([var]) \
+ asm qual (ALTERNATIVE("leaq %P[var], %%rsi; call this_cpu_cmpxchg16b_emu", \
+ "cmpxchg16b " __percpu_arg([var]), X86_FEATURE_CX16) \
: [var] "+m" (_var), \
"+a" (old__.low), \
"+d" (old__.high) \
: "b" (new__.low), \
"c" (new__.high) \
- : "memory"); \
+ : "memory", "rsi"); \
\
old__.var; \
})
@@ -400,12 +402,13 @@ do { \
bool __ret; \
typeof(pcp1) __o1 = (o1), __n1 = (n1); \
typeof(pcp2) __o2 = (o2), __n2 = (n2); \
- alternative_io("leaq %P1,%%rsi\n\tcall this_cpu_cmpxchg16b_emu\n\t", \
- "cmpxchg16b " __percpu_arg(1) "\n\tsetz %0\n\t", \
- X86_FEATURE_CX16, \
- ASM_OUTPUT2("=a" (__ret), "+m" (pcp1), \
- "+m" (pcp2), "+d" (__o2)), \
- "b" (__n1), "c" (__n2), "a" (__o1) : "rsi"); \
+ asm volatile (ALTERNATIVE("leaq %P1, %%rsi; call this_cpu_cmpxchg16b_emu", \
+ "cmpxchg16b " __percpu_arg(1), X86_FEATURE_CX16) \
+ "setz %0" \
+ : "=a" (__ret), "+m" (pcp1) \
+ : "b" (__n1), "c" (__n2), \
+ "a" (__o1), "d" (__o2) \
+ : "memory", "rsi"); \
__ret; \
})

--- a/arch/x86/lib/Makefile
+++ b/arch/x86/lib/Makefile
@@ -61,8 +61,9 @@ ifeq ($(CONFIG_X86_32),y)
lib-y += strstr_32.o
lib-y += string_32.o
lib-y += memmove_32.o
+ lib-y += cmpxchg8b_emu.o
ifneq ($(CONFIG_X86_CMPXCHG64),y)
- lib-y += cmpxchg8b_emu.o atomic64_386_32.o
+ lib-y += atomic64_386_32.o
endif
else
obj-y += iomap_copy_64.o
--- a/arch/x86/lib/cmpxchg16b_emu.S
+++ b/arch/x86/lib/cmpxchg16b_emu.S
@@ -1,47 +1,54 @@
/* SPDX-License-Identifier: GPL-2.0-only */
#include <linux/linkage.h>
#include <asm/percpu.h>
+#include <asm/processor-flags.h>

.text

/*
+ * Emulate 'cmpxchg16b %gs:(%rsi)'
+ *
* Inputs:
* %rsi : memory location to compare
* %rax : low 64 bits of old value
* %rdx : high 64 bits of old value
* %rbx : low 64 bits of new value
* %rcx : high 64 bits of new value
- * %al : Operation successful
+ *
+ * Notably this is not LOCK prefixed and is not safe against NMIs
*/
SYM_FUNC_START(this_cpu_cmpxchg16b_emu)

-#
-# Emulate 'cmpxchg16b %gs:(%rsi)' except we return the result in %al not
-# via the ZF. Caller will access %al to get result.
-#
-# Note that this is only useful for a cpuops operation. Meaning that we
-# do *not* have a fully atomic operation but just an operation that is
-# *atomic* on a single cpu (as provided by the this_cpu_xx class of
-# macros).
-#
pushfq
cli

- cmpq PER_CPU_VAR((%rsi)), %rax
- jne .Lnot_same
- cmpq PER_CPU_VAR(8(%rsi)), %rdx
- jne .Lnot_same
+ /* if (*ptr == old) */
+ cmpq PER_CPU_VAR(0(%rsi)), %rax
+ jne .Lnot_same
+ cmpq PER_CPU_VAR(8(%rsi)), %rdx
+ jne .Lnot_same
+
+ /* *ptr = new */
+ movq %rbx, PER_CPU_VAR(0(%rsi))
+ movq %rcx, PER_CPU_VAR(8(%rsi))

- movq %rbx, PER_CPU_VAR((%rsi))
- movq %rcx, PER_CPU_VAR(8(%rsi))
+ /* set ZF in EFLAGS to indicate success */
+ orl $X86_EFLAGS_ZF, (%rsp)

popfq
- mov $1, %al
RET

.Lnot_same:
+ /* *ptr != old */
+
+ /* old = *ptr */
+ movq PER_CPU_VAR(0(%rsi)), %rax
+ movq PER_CPU_VAR(8(%rsi)), %rdx
+
+ /* clear ZF in EFLAGS to indicate failure */
+ andl $(~X86_EFLAGS_ZF), (%rsp)
+
popfq
- xor %al,%al
RET

SYM_FUNC_END(this_cpu_cmpxchg16b_emu)
--- a/arch/x86/lib/cmpxchg8b_emu.S
+++ b/arch/x86/lib/cmpxchg8b_emu.S
@@ -2,10 +2,16 @@

#include <linux/linkage.h>
#include <asm/export.h>
+#include <asm/percpu.h>
+#include <asm/processor-flags.h>

.text

+#ifndef CONFIG_X86_CMPXCHG64
+
/*
+ * Emulate 'cmpxchg8b (%esi)' on UP
+ *
* Inputs:
* %esi : memory location to compare
* %eax : low 32 bits of old value
@@ -15,32 +21,61 @@
*/
SYM_FUNC_START(cmpxchg8b_emu)

-#
-# Emulate 'cmpxchg8b (%esi)' on UP except we don't
-# set the whole ZF thing (caller will just compare
-# eax:edx with the expected value)
-#
pushfl
cli

- cmpl (%esi), %eax
- jne .Lnot_same
- cmpl 4(%esi), %edx
- jne .Lhalf_same
+ cmpl 0(%esi), %eax
+ jne .Lnot_same
+ cmpl 4(%esi), %edx
+ jne .Lnot_same
+
+ movl %ebx, 0(%esi)
+ movl %ecx, 4(%esi)

- movl %ebx, (%esi)
- movl %ecx, 4(%esi)
+ orl $X86_EFLAGS_ZF, (%esp)

popfl
RET

.Lnot_same:
- movl (%esi), %eax
-.Lhalf_same:
- movl 4(%esi), %edx
+ movl 0(%esi), %eax
+ movl 4(%esi), %edx
+
+ andl $(~X86_EFLAGS_ZF), (%esp)

popfl
RET

SYM_FUNC_END(cmpxchg8b_emu)
EXPORT_SYMBOL(cmpxchg8b_emu)
+
+#endif
+
+SYM_FUNC_START(this_cpu_cmpxchg8b_emu)
+
+ pushfl
+ cli
+
+ cmpl PER_CPU_VAR(0(%esi)), %eax
+ jne .Lnot_same2
+ cmpl PER_CPU_VAR(4(%esi)), %edx
+ jne .Lnot_same2
+
+ movl %ebx, PER_CPU_VAR(0(%esi))
+ movl %ecx, PER_CPU_VAR(4(%esi))
+
+ orl $X86_EFLAGS_ZF, (%esp)
+
+ popfl
+ RET
+
+.Lnot_same2:
+ movl PER_CPU_VAR(0(%esi)), %eax
+ movl PER_CPU_VAR(4(%esi)), %edx
+
+ andl $(~X86_EFLAGS_ZF), (%esp)
+
+ popfl
+ RET
+
+SYM_FUNC_END(this_cpu_cmpxchg8b_emu)