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SubjectRe: [PATCH v2 5/9] x86, pmem: push fallback handling to arch code
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On Wed, 2015-08-26 at 21:34 +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@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@linutronix.de>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: "H. Peter Anvin" <hpa@zytor.com>
> Cc: Toshi Kani <toshi.kani@hp.com>
> Cc: Ross Zwisler <ross.zwisler@linux.intel.com>
> Cc: Christoph Hellwig <hch@lst.de>
> [hch: set ARCH_HAS_PMEM_API=n on X86_32]
> Signed-off-by: Dan Williams <dan.j.williams@intel.com>

Thanks for making this change! It looks good.

Reviewed-by: Toshi Kani <toshi.kani@hp.com>

I have one minor comment below:

> ---
> arch/x86/Kconfig | 2 +-
> arch/x86/include/asm/io.h | 2 --
> arch/x86/include/asm/pmem.h | 8 ++------
> drivers/acpi/nfit.c | 2 +-
> drivers/nvdimm/pmem.c | 2 +-
> include/linux/pmem.h | 28 +++++++++++++++++-----------
> 6 files changed, 22 insertions(+), 22 deletions(-)
>
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index 76c61154ed50..5912859df533 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -27,7 +27,7 @@ config X86
> select ARCH_HAS_ELF_RANDOMIZE
> select ARCH_HAS_FAST_MULTIPLIER
> select ARCH_HAS_GCOV_PROFILE_ALL
> - select ARCH_HAS_PMEM_API
> + select ARCH_HAS_PMEM_API if X86_64
> select ARCH_HAS_SG_CHAIN
> select ARCH_HAVE_NMI_SAFE_CMPXCHG
> select ARCH_MIGHT_HAVE_ACPI_PDC if ACPI
> diff --git a/arch/x86/include/asm/io.h b/arch/x86/include/asm/io.h
> index d241fbd5c87b..83ec9b1d77cc 100644
> --- a/arch/x86/include/asm/io.h
> +++ b/arch/x86/include/asm/io.h
> @@ -248,8 +248,6 @@ static inline void flush_write_buffers(void)
> #endif
> }
>
> -#define ARCH_MEMREMAP_PMEM MEMREMAP_WB

Should it be better to do:

#else /* !CONFIG_ARCH_HAS_PMEM_API */
#define ARCH_MEMREMAP_PMEM MEMREMAP_WT

so that you can remove all '#ifdef ARCH_MEMREMAP_PMEM' stuff?

Thanks,
-Toshi


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