lkml.org 
[lkml]   [2015]   [Jan]   [24]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH v2 0/2] add support for new persistent memory instructions
On Fri, Jan 23, 2015 at 03:03:41PM -0800, H. Peter Anvin wrote:
> For the specific case of CLWB, we can use an "m" input rather than a
> "+m" output, simply because CLWB (or CLFLUSH* used as a standin for CLWB
> doesn't need to be ordered with respect to loads (whereas CLFLUSH* do).

Well, we could do something like:

volatile struct { char x[64]; } *p = __p;

if (static_cpu_has(X86_FEATURE_CLWB))
asm volatile(".byte 0x66,0x0f,0xae,0x30" :: "m" (*p), "a" (p));
else
asm volatile(ALTERNATIVE(
".byte 0x3e; clflush (%[pax])",
".byte 0x66; clflush (%[pax])", /* clflushopt (%%rax) */
X86_FEATURE_CLFLUSHOPT)
: [p] "+m" (*p)
: [pax] "a" (p));

which would simplify the alternative macro too.

Generated asm looks ok to me (my objdump doesn't know CLWB yet :)):

0000000000000aa0 <myclflush>:
aa0: 55 push %rbp
aa1: 48 89 e5 mov %rsp,%rbp
aa4: eb 0a jmp ab0 <myclflush+0x10>
aa6: 48 89 f8 mov %rdi,%rax
aa9: 66 0f ae 30 data16 xsaveopt (%rax)
aad: 5d pop %rbp
aae: c3 retq
aaf: 90 nop
ab0: 48 89 f8 mov %rdi,%rax
ab3: 3e 0f ae 38 clflush %ds:(%rax)
ab7: 5d pop %rbp
ab8: c3 retq

> Should we use an SFENCE as a standin if pcommit is unavailable, in case
> we end up using CLFLUSHOPT?

Btw, is PCOMMIT a lightweight SFENCE for this persistent memory aspect
to make sure stuff has become persistent after executing it? But not all
stuff like SFENCE so SFENCE is the bigger hammer?

--
Regards/Gruss,
Boris.

ECO tip #101: Trim your mails when you reply.
--


\
 
 \ /
  Last update: 2015-01-24 12:21    [W:0.099 / U:0.328 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site