Re: [PATCH v3 1/2] x86: Add support for the pcommit instruction

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

On 01/27/2015 08:53 AM, Ross Zwisler wrote:
> Add support for the new pcommit (persistent commit) 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 pcommit instruction ensures that data that has been flushed from the
> processor's cache hierarchy with clwb, clflushopt or clflush is accepted to
> memory and is durable on the DIMM.  The primary use case for this is persistent
> memory.
> 
> 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.
> 
> Pcommit must always be ordered by an mfence or sfence, so to help
> simplify things we include both the pcommit and the required sfence in
> the alternatives generated by pcommit_sfence().  The other option is to
> keep them separated, but on platforms that don't support pcommit this
> would then turn into:
> 
> 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();
> 
>         nop(); /* from pcommit(), via alternatives */
> 
>         /*
>          * sfence to order pcommit
>          * mfence via mb() also works
>          */
>         wmb();
> }
> 
> This is still correct, but now you've got two fences separated by only a
> nop.  With the commit and the fence together in pcommit_sfence() you
> avoid the final unneeded fence.

Acked-by: H. Peter Anvin <hpa@linux.intel.com>