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




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