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From: "H. Peter Anvin" <h.peter.anvin@intel.com>
To: Ross Zwisler <ross.zwisler@linux.intel.com>,
	linux-kernel@vger.kernel.org
Cc: Ingo Molnar <mingo@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Borislav Petkov <bp@alien8.de>
Subject: Re: [PATCH v2 0/2] add support for new persistent memory instructions
Date: Fri, 23 Jan 2015 15:03:41 -0800	[thread overview]
Message-ID: <54C2D34D.7010709@intel.com> (raw)
In-Reply-To: <1422045628-16225-1-git-send-email-ross.zwisler@linux.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



  parent reply	other threads:[~2015-01-23 23:03 UTC|newest]

Thread overview: 12+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2015-01-23 20:40 Ross Zwisler
2015-01-23 20:40 ` [PATCH v2 1/2] x86: Add support for the pcommit instruction Ross Zwisler
2015-01-23 20:40 ` [PATCH v2 2/2] x86: Add support for the clwb instruction Ross Zwisler
2015-01-23 23:03 ` H. Peter Anvin [this message]
2015-01-24 11:14   ` [PATCH v2 0/2] add support for new persistent memory instructions Borislav Petkov
2015-01-26 19:59     ` Ross Zwisler
2015-01-26 21:34       ` Borislav Petkov
2015-01-26 21:50         ` Ross Zwisler
2015-01-26 22:39           ` Borislav Petkov
2015-01-26 23:14             ` Ross Zwisler
2015-01-26 19:51   ` Ross Zwisler
2015-01-26 20:05     ` H. Peter Anvin

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