linux-kernel - Re: [tip:x86/asm] x86: Add support for the clwb instruction

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Date:	Fri, 3 Apr 2015 07:04:14 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Ross Zwisler <ross.zwisler@...ux.intel.com>
Cc:	tglx@...utronix.de, torvalds@...ux-foundation.org,
	linux-kernel@...r.kernel.org, bp@...e.de, hpa@...ux.intel.com,
	hpa@...or.com, linux-tip-commits@...r.kernel.org
Subject: Re: [tip:x86/asm] x86: Add support for the clwb instruction


* Ross Zwisler <ross.zwisler@...ux.intel.com> wrote:

> On Wed, 2015-02-18 at 16:29 -0800, tip-bot for Ross Zwisler wrote:
> > Commit-ID:  3b68983dc66c61da3ab4191b891084a7ab09e3e1
> > Gitweb:     http://git.kernel.org/tip/3b68983dc66c61da3ab4191b891084a7ab09e3e1
> > Author:     Ross Zwisler <ross.zwisler@...ux.intel.com>
> > AuthorDate: Tue, 27 Jan 2015 09:53:51 -0700
> > Committer:  Ingo Molnar <mingo@...nel.org>
> > CommitDate: Thu, 19 Feb 2015 00:06:38 +0100
> > 
> > x86: Add support for the clwb instruction
> > 
> > Add support for the new clwb (cache line write back)
> > instruction.  This instruction was 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
> > 
> > The clwb instruction is used to write back the contents of
> > dirtied cache lines to memory without evicting the cache lines
> > from the processor's cache hierarchy.  This should be used in
> > favor of clflushopt or clflush in cases where you require the
> > cache line to be written to memory but plan to access the data
> > again in the near future.
> > 
> > One of the main use cases for this is with persistent memory
> > where clwb can be used with pcommit to ensure that data has been
> > accepted to memory and is durable on the DIMM.
> > 
> > This function shows how to properly use clwb/clflushopt/clflush
> > and pcommit with appropriate fencing:
> > 
> > void flush_and_commit_buffer(void *vaddr, unsigned int size)
> > {
> > 	void *vend = vaddr + size - 1;
> > 
> > 	for (; vaddr < vend; vaddr += boot_cpu_data.x86_clflush_size)
> > 		clwb(vaddr);
> > 
> > 	/* Flush any possible final partial cacheline */
> > 	clwb(vend);
> > 
> > 	/*
> > 	 * sfence to order clwb/clflushopt/clflush cache flushes
> > 	 * mfence via mb() also works
> > 	 */
> > 	wmb();
> > 
> > 	/* pcommit and the required sfence for ordering */
> > 	pcommit_sfence();
> > }
> > 
> > After this function completes the data pointed to by vaddr is
> > has been accepted to memory and will be durable if the vaddr
> > points to persistent memory.
> > 
> > Regarding the details of how the alternatives assembly is set
> > up, we need one additional byte at the beginning of the clflush
> > so that we can flip it into a clflushopt by changing that byte
> > into a 0x66 prefix.  Two options are to either insert a 1 byte
> > ASM_NOP1, or to add a 1 byte NOP_DS_PREFIX.  Both have no
> > functional effect with the plain clflush, but I've been told
> > that executing a clflush + prefix should be faster than
> > executing a clflush + NOP.
> > 
> > We had to hard code the assembly for clwb because, lacking the
> > ability to assemble the clwb instruction itself, the next
> > closest thing is to have an xsaveopt instruction with a 0x66
> > prefix.  Unfortunately xsaveopt itself is also relatively new,
> > and isn't included by all the GCC versions that the kernel needs
> > to support.
> > 
> > Signed-off-by: Ross Zwisler <ross.zwisler@...ux.intel.com>
> > Acked-by: Borislav Petkov <bp@...e.de>
> > Acked-by: H. Peter Anvin <hpa@...ux.intel.com>
> > Cc: Linus Torvalds <torvalds@...ux-foundation.org>
> > Cc: Thomas Gleixner <tglx@...utronix.de>
> > Link: http://lkml.kernel.org/r/1422377631-8986-3-git-send-email-ross.zwisler@linux.intel.com
> > Signed-off-by: Ingo Molnar <mingo@...nel.org>
> 
> Ping on this patch - it looks like the pcommit patch is in the tip tree,
> but this one is missing?

Yeah, I applied it initially, then had some reservations about it - 
but those are now resolved so I've applied it to tip:x86/asm again.

Thanks,

	Ingo
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