Re: [PATCH v2] x86, uaccess: introduce copy_from_iter_wt for pmem / writethrough operations

From: Dan Williams
Date: Fri May 05 2017 - 10:12:09 EST

On Thu, May 4, 2017 at 11:54 PM, Ingo Molnar <mingo@xxxxxxxxxx> wrote:
> * Dan Williams <dan.j.williams@xxxxxxxxx> wrote:
>> The pmem driver has a need to transfer data with a persistent memory
>> destination and be able to rely on the fact that the destination writes
>> are not cached. It is sufficient for the writes to be flushed to a
>> cpu-store-buffer (non-temporal / "movnt" in x86 terms), as we expect
>> userspace to call fsync() to ensure data-writes have reached a
>> power-fail-safe zone in the platform. The fsync() triggers a REQ_FUA or
>> REQ_FLUSH to the pmem driver which will turn around and fence previous
>> writes with an "sfence".
>> Implement a __copy_from_user_inatomic_wt, memcpy_page_wt, and memcpy_wt,
>> that guarantee that the destination buffer is not dirty in the cpu cache
>> on completion. The new copy_from_iter_wt and sub-routines will be used
>> to replace the "pmem api" (include/linux/pmem.h +
>> arch/x86/include/asm/pmem.h). The availability of copy_from_iter_wt()
>> and memcpy_wt() are gated by the CONFIG_ARCH_HAS_UACCESS_WT config
>> symbol, and fallback to copy_from_iter_nocache() and plain memcpy()
>> otherwise.
>> This is meant to satisfy the concern from Linus that if a driver wants
>> to do something beyond the normal nocache semantics it should be
>> something private to that driver [1], and Al's concern that anything
>> uaccess related belongs with the rest of the uaccess code [2].
>> [1]:
>> [2]:
>> Cc: <x86@xxxxxxxxxx>
>> Cc: Jan Kara <jack@xxxxxxx>
>> Cc: Jeff Moyer <jmoyer@xxxxxxxxxx>
>> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
>> Cc: Christoph Hellwig <hch@xxxxxx>
>> Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
>> Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
>> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
>> Cc: Matthew Wilcox <mawilcox@xxxxxxxxxxxxx>
>> Cc: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx>
>> Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
>> ---
>> Changes since the initial RFC:
>> * s/writethru/wt/ since we already have ioremap_wt(), set_memory_wt(),
>> etc. (Ingo)
> Looks good to me. I suspect you'd like to carry this in the nvdimm tree?
> Acked-by: Ingo Molnar <mingo@xxxxxxxxxx>

Thanks, Ingo!. Yes, I'll carry it in nvdimm.git for 4.13.