linux-kernel - Re: [PATCH v3 01/14] x86, uaccess: introduce copy_from_iter_flushcache for pmem / cache-bypass operations

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Date:   Sun, 18 Jun 2017 19:02:54 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Christoph Hellwig <hch@....de>
Cc:     "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        Jan Kara <jack@...e.cz>, dm-devel@...hat.com,
        Toshi Kani <toshi.kani@....com>,
        Matthew Wilcox <mawilcox@...rosoft.com>,
        X86 ML <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Jeff Moyer <jmoyer@...hat.com>, Ingo Molnar <mingo@...hat.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        "H. Peter Anvin" <hpa@...or.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>
Subject: Re: [PATCH v3 01/14] x86, uaccess: introduce copy_from_iter_flushcache
 for pmem / cache-bypass operations

On Sun, Jun 18, 2017 at 1:28 AM, Christoph Hellwig <hch@....de> wrote:
> On Fri, Jun 09, 2017 at 01:23:51PM -0700, Dan Williams wrote:
>> Implement a __copy_from_user_inatomic_flushcache, memcpy_page_flushcache, and
>> memcpy_flushcache, that guarantee that the destination buffer is not dirty in
>> the cpu cache on completion. The new copy_from_iter_flushcache and sub-routines
>
> Wouldn't writethrough be a better name?

I started with _writethrough, Ingo suggested _wt, and then Toshi
rightly pointed out that _wt might lead applications to assume that
the involved cache-lines are valid on return which may not be true. So
we settled on _flushcache in this thread [1].

[1]: https://lkml.org/lkml/2017/5/6/99

>> will be used to replace the "pmem api" (include/linux/pmem.h +
>> arch/x86/include/asm/pmem.h). The availability of copy_from_iter_flushcache()
>> and memcpy_flushcache() are gated by the CONFIG_ARCH_HAS_UACCESS_FLUSHCACHE
>> config symbol, and fallback to copy_from_iter_nocache() and plain memcpy()
>> otherwise.
>
> What is UACCESS about memcpy_flushcache?

The uaccess part comes from the fact that the conversion provides all
the _flushcache versions of the copy_from_iter() operations
(__copy_from_user_flushcache, memcpy_page_flushcache,
memcpy_flushcache). It also stems from Al asking that
__copy_user_flushcache() live in arch/x86/lib/usercopy_64.c. That
said, I wouldn't object to a different name for the config symbol.