linux-kernel - Re: [PATCH v2] x86, uaccess: introduce copy_from_iter

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Date:   Fri, 5 May 2017 22:44:40 +0000
From:   "Kani, Toshimitsu" <toshi.kani@....com>
To:     "dan.j.williams@...el.com" <dan.j.williams@...el.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "jmoyer@...hat.com" <jmoyer@...hat.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "hch@....de" <hch@....de>,
        "viro@...iv.linux.org.uk" <viro@...iv.linux.org.uk>,
        "x86@...nel.org" <x86@...nel.org>,
        "mawilcox@...rosoft.com" <mawilcox@...rosoft.com>,
        "hpa@...or.com" <hpa@...or.com>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "ross.zwisler@...ux.intel.com" <ross.zwisler@...ux.intel.com>,
        "jack@...e.cz" <jack@...e.cz>
Subject: Re: [PATCH v2] x86, uaccess: introduce copy_from_iter_wt for pmem /
 writethrough operations

On Fri, 2017-05-05 at 15:25 -0700, Dan Williams wrote:
> On Fri, May 5, 2017 at 1:39 PM, Kani, Toshimitsu <toshi.kani@....com>
> wrote:
 :
> > > ---
> > > 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.

I see.  I agree that its behavior is different from the existing one
with "_nocache".   That said, I think "wt" or "write-through" generally
means that writes allocate cachelines and keep them clean by writing to
memory.  So, subsequent reads to the destination will hit the
cachelines.  This is not the case with this interface.

Thanks,
-Toshi