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

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Date:	Sun, 28 Feb 2010 20:17:06 +0100
From:	Pavel Machek <pavel@....cz>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	Russell King - ARM Linux <linux@....linux.org.uk>,
	Catalin Marinas <catalin.marinas@....com>,
	Matthew Dharm <mdharm-kernel@...-eyed-alien.net>,
	linux-usb@...r.kernel.org, "Mankad,Maulik Ojas" <x0082077@...com>,
	Sergei Shtylyov <sshtylyov@...mvista.com>,
	Ming Lei <tom.leiming@...il.com>,
	Sebastian Siewior <bigeasy@...utronix.de>,
	Oliver Neukum <oliver@...kum.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	"Shilimkar,Santosh" <santosh.shilimkar@...com>,
	Greg KH <greg@...ah.com>,
	linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
	James Bottomley <James.Bottomley@...senPartnership.com>
Subject: Re: USB mass storage and ARM cache coherency

> There's two potential problems with the approach, and maybe more that I
> have missed though. One is the case of a networked filesystem where the
> executable pages are modified remotely. However, I would expect such a
> program to invalidate the PTE mappings before making the change visible,
> so we -do- get a chance to re-flush provided something clears PG_arch_1.
> 
> Then, there's In the case of a multithread app, where one thread does
> the cache flush and another thread then executes, the earlier ARMs
> without broadcast ops have a potential problem there. In fact, some
> variant of PowerPC 440 have the same problem and some people are
> (ab)using those for SMP setups I'm being told.
> 
> For that case, I see two options. One is a big hammer but would make
> existing code work to "most" extent: Don't allow a page to be both
> writable and executable. Ping-pong the page permission lazily and flush
> when transitioning from write to exec.
> 
> That means using a spare bit for Linux _PAGE_RW separate from your real
> RW bit I suppose, since you have HW loaded PTEs (on 440 it's easier
> since we SW load, we can do the fixup there, though it has a perf impact
> obviously).
> 
> Another option would be to make some syscall mandatory to "sync" caches
> which could then do IPIs or whatever else is needed. But that would
> require changing existing userspace code.

Or you could do first option by default, and add mmap flag that says
that application is responsible for cross-cpu cache flushes...?
								Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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