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

[Dan Williams]

On Mon, May 8, 2017 at 1:32 PM, Ross Zwisler
<ross.zwisler@xxxxxxxxxxxxxxx> wrote:
> On Fri, Apr 28, 2017 at 12:39:12PM -0700, Dan Williams 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]: https://lists.01.org/pipermail/linux-nvdimm/2017-January/008364.html
>> [2]: https://lists.01.org/pipermail/linux-nvdimm/2017-April/009942.html
>>
>> 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>
[..]
> I took a pretty hard look at the changes in arch/x86/lib/usercopy_64.c, and
> they look correct to me.  The inline assembly for non-temporal copies mixed
> with C for loop control is IMHO much easier to follow than the pure assembly
> of __copy_user_nocache().
>
> Reviewed-by: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx>

Thanks Ross, I appreciate it.