Re: [PATCH v2 5/9] x86, pmem: push fallback handling to arch code

[Ross Zwisler]

On Wed, Aug 26, 2015 at 09:34:20PM +0000, Williams, Dan J wrote:
> On Wed, 2015-08-26 at 14:41 +0200, Christoph Hellwig wrote:
> > I like the intent behind this, but not the implementation.
> > 
> > I think the right approach is to keep the defaults in linux/pmem.h
> > and simply not set CONFIG_ARCH_HAS_PMEM_API for x86-32.
> 
> Yes, that makes things much cleaner.  Revised patch and changelog below:
> 
> 8<----
> Subject: x86, pmem: clarify that ARCH_HAS_PMEM_API implies PMEM mapped WB
> 
> From: Dan Williams <dan.j.williams@xxxxxxxxx>
> 
> Given that a write-back (WB) mapping plus non-temporal stores is
> expected to be the most efficient way to access PMEM, update the
> definition of ARCH_HAS_PMEM_API to imply arch support for
> WB-mapped-PMEM.  This is needed as a pre-requisite for adding PMEM to
> the direct map and mapping it with struct page.
> 
> The above clarification for X86_64 means that memcpy_to_pmem() is
> permitted to use the non-temporal arch_memcpy_to_pmem() rather than
> needlessly fall back to default_memcpy_to_pmem() when the pcommit
> instruction is not available.  When arch_memcpy_to_pmem() is not
> guaranteed to flush writes out of cache, i.e. on older X86_32
> implementations where non-temporal stores may just dirty cache,
> ARCH_HAS_PMEM_API is simply disabled.
> 
> The default fall back for persistent memory handling remains.  Namely,
> map it with the WT (write-through) cache-type and hope for the best.
> 
> arch_has_pmem_api() is updated to only indicate whether the arch
> provides the proper helpers to meet the minimum "writes are visible
> outside the cache hierarchy after memcpy_to_pmem() + wmb_pmem()".  Code
> that cares whether wmb_pmem() actually flushes writes to pmem must now
> call arch_has_wmb_pmem() directly.
> 
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
> Cc: Toshi Kani <toshi.kani@xxxxxx>
> Cc: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx>
> Cc: Christoph Hellwig <hch@xxxxxx>
> [hch: set ARCH_HAS_PMEM_API=n on X86_32]
> Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>

Yep, this seems like a good change.

Reviewed-by: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx>
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