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:
Subject: x86, pmem: clarify that ARCH_HAS_PMEM_API implies PMEM mapped WB
From: Dan Williams <dan.j.williams(a)intel.com>
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(a)linutronix.de>
Cc: Ingo Molnar <mingo(a)redhat.com>
Cc: "H. Peter Anvin" <hpa(a)zytor.com>
Cc: Toshi Kani <toshi.kani(a)hp.com>
Cc: Ross Zwisler <ross.zwisler(a)linux.intel.com>
Cc: Christoph Hellwig <hch(a)lst.de>
[hch: set ARCH_HAS_PMEM_API=n on X86_32]
Signed-off-by: Dan Williams <dan.j.williams(a)intel.com>
Yep, this seems like a good change.
Reviewed-by: Ross Zwisler <ross.zwisler(a)linux.intel.com>