On Thu, Jan 07, 2016 at 01:05:35PM -0800, H. Peter Anvin wrote:
> On 01/07/16 11:44, Chris Wilson wrote:
> >
> > Now I feel silly. Looking at the .s, there is no difference with the
> > addition of the barrier to clflush_cache_range(). And sure enough
> > letting the test run for longer, we see a failure. I fell for a placebo.
> >
> > The failing assertion is always on the last cacheline and is always one
> > value behind. Oh well, back to wondering where we miss the flush.
> > -Chris
> >
>
> Could you include the assembly here?
Sure, here you go:
.LHOTB18:
.p2align 4,,15
.globl clflush_cache_range
.type clflush_cache_range, @function
clflush_cache_range:
.LFB2505:
.loc 1 131 0
.cfi_startproc
.LVL194:
1: call __fentry__
.loc 1 132 0
movzwl boot_cpu_data+198(%rip), %eax
.loc 1 131 0
pushq %rbp
.cfi_def_cfa_offset 16
.cfi_offset 6, -16
.loc 1 133 0
movl %esi, %esi
.LVL195:
addq %rdi, %rsi
.LVL196:
.loc 1 131 0
movq %rsp, %rbp
.cfi_def_cfa_register 6
.loc 1 132 0
subl $1, %eax
cltq
.LVL197:
.loc 1 136 0
#APP
# 136 "arch/x86/mm/pageattr.c" 1
mfence
# 0 "" 2
.loc 1 138 0
#NO_APP
notq %rax
.LVL198:
andq %rax, %rdi
.LVL199:
cmpq %rdi, %rsi
jbe .L216
.L217:
.LBB1741:
.LBB1742:
.loc 8 198 0
#APP
# 198 "./arch/x86/include/asm/special_insns.h" 1
661:
.byte 0x3e; clflush (%rdi)
662:
.skip -(((6651f-6641f)-(662b-661b)) > 0) * ((6651f-6641f)-(662b-661b)),0x90
663:
.pushsection .altinstructions,"a"
.long 661b - .
.long 6641f - .
.word ( 9*32+23)
.byte 663b-661b
.byte 6651f-6641f
.byte 663b-662b
.popsection
.pushsection .altinstr_replacement, "ax"
6641:
.byte 0x66; clflush (%rdi)
6651:
.popsection
# 0 "" 2
#NO_APP
.LBE1742:
.LBE1741:
.loc 1 141 0
.loc 1 139 0
movzwl boot_cpu_data+198(%rip), %eax
addq %rax, %rdi
.loc 1 138 0
cmpq %rdi, %rsi
ja .L217
.L216:
.loc 1 144 0
#APP
# 144 "arch/x86/mm/pageattr.c" 1
mfence
# 0 "" 2
.loc 1 145 0
#NO_APP
popq %rbp
.cfi_restore 6
.cfi_def_cfa 7, 8
ret
.cfi_endproc
.LFE2505:
.size clflush_cache_range, .-clflush_cache_range
.section .text.unlikely
Whilst you are looking at this asm, note that we reload
boot_cpu_data.x86_cflush_size everytime around the loop. That's a small
but noticeable extra cost (especially when we are only flushing a single
cacheline).
diff --git a/arch/x86/mm/pageattr.c b/arch/x86/mm/pageattr.c
index a3137a4..2cd2b4b 100644
--- a/arch/x86/mm/pageattr.c
+++ b/arch/x86/mm/pageattr.c
@@ -129,14 +129,13 @@ within(unsigned long addr, unsigned long start, unsigned long end)
*/
void clflush_cache_range(void *vaddr, unsigned int size)
{
- unsigned long clflush_mask = boot_cpu_data.x86_clflush_size - 1;
+ unsigned long clflush_size = boot_cpu_data.x86_clflush_size;
void *vend = vaddr + size;
- void *p;
+ void *p = (void *)((unsigned long)vaddr & ~(clflush_size - 1));
mb();
- for (p = (void *)((unsigned long)vaddr & ~clflush_mask);
- p < vend; p += boot_cpu_data.x86_clflush_size)
+ for (; p < vend; p += clflush_size)
clflushopt(p);
mb();
-Chris
--
Chris Wilson, Intel Open Source Technology Centre
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