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

From: "Kani, Toshimitsu" <toshi.kani@hpe.com>
To: Dan Williams <dan.j.williams@intel.com>
Cc: "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"linux-block@vger.kernel.org" <linux-block@vger.kernel.org>,
	"jmoyer@redhat.com" <jmoyer@redhat.com>,
	"tglx@linutronix.de" <tglx@linutronix.de>,
	"hch@lst.de" <hch@lst.de>,
	"viro@zeniv.linux.org.uk" <viro@zeniv.linux.org.uk>,
	"x86@kernel.org" <x86@kernel.org>,
	"mawilcox@microsoft.com" <mawilcox@microsoft.com>,
	"hpa@zytor.com" <hpa@zytor.com>,
	"linux-nvdimm@lists.01.org" <linux-nvdimm@ml01.01.org>,
	"mingo@redhat.com" <mingo@redhat.com>,
	"linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
	"ross.zwisler@linux.intel.com" <ross.zwisler@linux.intel.com>,
	"jack@suse.cz" <jack@suse.cz>
Subject: RE: [PATCH v2] x86, uaccess: introduce copy_from_iter_wt for pmem / writethrough operations
Date: Sat, 6 May 2017 03:17:19 +0000	[thread overview]
Message-ID: <AT5PR84MB02607F504E354866D8762CD182E80@AT5PR84MB0260.NAMPRD84.PROD.OUTLOOK.COM> (raw)
In-Reply-To: <CAPcyv4iHGAGfb1ZUDYovUKs1+tr48CxEbngCHE-QiSFXAjeChA@mail.gmail.com>

> On Fri, May 5, 2017 at 3:44 PM, Kani, Toshimitsu <toshi.kani@hpe.com>
> wrote:
> > 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@hpe.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.
> 
> True... maybe _nocache_strict()? Or, leave it _wt() until someone
> comes along and is surprised that the cache is not warm for reads
> after memcpy_wt(), at which point we can ask "why not just use plain
> memcpy then?", or set the page-attributes to WT.

I prefer _nocache_strict(), if it's not too long, since it avoids any
confusion.  If other arches actually implement it with WT semantics,
we might become the one to change it, instead of the caller.

Thanks,
-Toshi