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

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Date:	Thu, 04 Mar 2010 15:25:23 +0000
From:	Catalin Marinas <catalin.marinas@....com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	James Bottomley <James.Bottomley@...senPartnership.com>,
	Pavel Machek <pavel@....cz>,
	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, 2010-03-04 at 14:27 +0000, Russell King - ARM Linux wrote:
> 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.

The I-cache flushing is already handled in update_mmu_cache (or
set_pte_at in a future patch; I'm not talking about other issues on
ARM11MPCore here).

We always invalidate the I-cache currently (since we may have DMA
transfers and the page's D-cache is clean). As an optimisation, we could
use PG_arch_2 for I-cache but I don't think there is much performance
benefit compared to always invalidating the I-cache flushing.

My understanding from this long discussion is that we cannot get the
kernel modifying a page cache page which is already mapped in user space
(well, ptrace does this but we flush the cache there already).

-- 
Catalin

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