Re: [PATCH v2 0/2] add support for new persistent memory instructions

["H. Peter Anvin" <h.peter.anvin@intel.com>]

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