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

From: Dan Williams
Date: Fri May 05 2017 - 18:25:10 EST

On Fri, May 5, 2017 at 1:39 PM, Kani, Toshimitsu <toshi.kani@xxxxxxx> wrote:
> On Fri, 2017-04-28 at 12:39 -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]:
>> html
>> [2]:
>> ml
>> 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)
> Sorry I should have said earlier, but I think the term "wt" is
> misleading. Non-temporal stores used in memcpy_wt() provide WC
> semantics, not WT semantics.

The non-temporal stores do, but memcpy_wt() is using a combination of
non-temporal stores and explicit cache flushing.

> How about using "nocache" as it's been
> used in __copy_user_nocache()?

The difference in my mind is that the "_nocache" suffix indicates
opportunistic / optional cache pollution avoidance whereas "_wt"
strictly arranges for caches not to contain dirty data upon completion
of the routine. For example, non-temporal stores on older x86 cpus
could potentially leave dirty data in the cache, so memcpy_wt on those
cpus would need to use explicit cache flushing.