USB mass storage and ARM cache coherency

[James Bottomley]

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.

James