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.
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