Messages in this thread | ![/](/images/icornerl.gif) | | Date | Fri, 23 Jan 2015 15:03:41 -0800 | From | "H. Peter Anvin" <> | Subject | Re: [PATCH v2 0/2] add support for new persistent memory instructions |
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On 01/23/2015 12:40 PM, Ross Zwisler wrote: > This patch set adds support for two new persistent memory instructions, pcommit > and clwb. These instructions were announced in the document "Intel > Architecture Instruction Set Extensions Programming Reference" with reference > number 319433-022. > > https://software.intel.com/sites/default/files/managed/0d/53/319433-022.pdf >
Please explain in these patch descriptions what the instructions actually do.
+ volatile struct { char x[64]; } *p = __p; + + asm volatile(ALTERNATIVE_2( + ".byte " __stringify(NOP_DS_PREFIX) "; clflush (%[pax])", + ".byte 0x66; clflush (%[pax])", /* clflushopt (%%rax) */ + X86_FEATURE_CLFLUSHOPT, + ".byte 0x66, 0x0f, 0xae, 0x30", /* clwb (%%rax) */ + X86_FEATURE_CLWB) + : [p] "+m" (*p) + : [pax] "a" (p));
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).
Now, one can argue that for performance reasons we should should still use "+m" in case we use the CLFLUSH* standin, to avoid flushing a cache line to memory just to bring it back in.
+static inline void pcommit(void) +{ + alternative(ASM_NOP4, ".byte 0x66, 0x0f, 0xae, 0xf8", + X86_FEATURE_PCOMMIT); +} +
Should we use an SFENCE as a standin if pcommit is unavailable, in case we end up using CLFLUSHOPT?
-hpa
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