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

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Message-ID: <20100304142704.GB6622@n2100.arm.linux.org.uk>
Date:	Thu, 4 Mar 2010 14:27:04 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	James Bottomley <James.Bottomley@...senPartnership.com>
Cc:	Pavel Machek <pavel@....cz>,
	Catalin Marinas <catalin.marinas@....com>,
	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>,
	benh@...nel.crashing.org, mdharm-kernel@...-eyed-alien.net,
	linux-usb@...r.kernel.org, x0082077@...com,
	sshtylyov@...mvista.com, tom.leiming@...il.com,
	bigeasy@...utronix.de, oliver@...kum.org,
	linux-kernel@...r.kernel.org, santosh.shilimkar@...com,
	greg@...ah.com, linux-arm-kernel@...ts.infradead.org
Subject: Re: USB mass storage and ARM cache coherency

On Thu, Mar 04, 2010 at 07:51:52PM +0530, James Bottomley wrote:
> On Thu, 2010-03-04 at 14:51 +0100, Pavel Machek wrote:
> > Seems like ARM has requirement other architectures do not, that is
> > a) not documented anywhere
> > b) causes problems
> > 
> > You could argue that performance improvement (how big is it, anyway?)
> > is worth it, but this should be agreed to by wider community...
> 
> Performance is always worth it provided we don't sacrifice correctness.
> The thing which was discovered in this thread is basically that ARM is
> handling deferred flushing (for D/I coherency) in a slightly different
> way from everyone else ... once that's fixed, ARM will likely not have
> the D/I problem, but we'll still have the libata (and other PIO systems)
> D flushing issue.

I think you've got that backwards.

Reversing the meaning of PG_arch_1 will probably fix the D aliasing issue -
since we'll interpret '0' to mean "page is dirty, it needs flushing before
hitting userspace", whereas '1' means "page has been cleaned; there are no
aliases."

This doesn not address the I/D coherency issue, where the Icache needs
attention to get rid of speculatively loaded cache lines while old data
was present in the cache.
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