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Subject: Re: USB mass storage and ARM cache coherency
From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
To: James Bottomley <James.Bottomley@HansenPartnership.com>
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Date: Thu, 04 Mar 2010 13:00:19 +1100
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On Wed, 2010-03-03 at 11:10 +0530, James Bottomley wrote:
> On Wed, 2010-03-03 at 16:10 +1100, Benjamin Herrenschmidt wrote:
> > On Wed, 2010-03-03 at 12:47 +0900, FUJITA Tomonori wrote:
> > > The ways to improve the approach (introducing PG_arch_2 or marking a
> > > page clean on dma_unmap_* with DMA_FROM_DEVICE like ia64 does) is up
> > > to architectures. 
> > 
> > How does the above work ? IE, the dma unmap will flush the D side but
> > not the I side ... or is the ia64 flush primitive magic enough to do
> > both ?
> 
> The point is that in a well regulated system, the I cache shouldn't need
> extra flushing in the kernel.  We should only be faulting in R-X pages.
> If we're operating on RWX pages (i.e. self modifying code), it's the job
> of userspace to keep I/D coherency.
> 
> So the only case the kernel needs to worry about is the R-X fault case
> for executable text code.

Still, you do need to flush I when a page cache page is recycled.

Cheers,
Ben.