linux-kernel - Re: USB mass storage and ARM cache coherency

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Message-ID: <20100301110610.GA16604@n2100.arm.linux.org.uk>
Date:	Mon, 1 Mar 2010 11:06:10 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Catalin Marinas <catalin.marinas@....com>
Cc:	James Bottomley <James.Bottomley@...senPartnership.com>,
	Matthew Dharm <mdharm-kernel@...-eyed-alien.net>,
	Oliver Neukum <oliver@...kum.org>, Greg KH <greg@...ah.com>,
	"Mankad, Maulik Ojas" <x0082077@...com>,
	Sergei Shtylyov <sshtylyov@...mvista.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Sebastian Siewior <bigeasy@...utronix.de>,
	linux-usb@...r.kernel.org,
	linux-kernel <linux-kernel@...r.kernel.org>,
	"Shilimkar, Santosh" <santosh.shilimkar@...com>,
	Pavel Machek <pavel@....cz>, Ming Lei <tom.leiming@...il.com>,
	linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>
Subject: Re: USB mass storage and ARM cache coherency

On Mon, Mar 01, 2010 at 10:39:14AM +0000, Catalin Marinas wrote:
> On Sun, 2010-02-28 at 05:01 +0000, James Bottomley wrote:
> > But the point of all of this is that I cache invalidation doesn't appear
> > anywhere in the I/O path ... so  if we're getting I/D incoherency,
> > there's some problem in the mm code (or there's a missing arch
> > assumption ... like I cache gets moved in more aggressively than we
> > expect).  Parisc is very sensitive to I/D incoherency, so we'd notice if
> > there were a serious generic problem here.
> 
> On ARM PIPT, it's probably because flush_cache_page isn't implemented.
> But as I said above, given the speculative fetches I don't think it
> would help much (well, it would work a bit better but not a complete
> fix).

Not quite.  flush_cache_page() is called when we unmap or replace a page
in userspace, which is completely the wrong place to do I-cache coherency
when you have speculatively loaded caches - or even D-cache coherency if
your cache behaves as a speculatively loaded PIPT or non-aliasing VIPT.

Flushing the I-cache after a page has been in userspace does nothing to
ensure that there aren't any I-cache lines associated with that page
when you next come to map it into userspace.
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