Re: USB mass storage and ARM cache coherency

[Pavel Machek <pavel@ucw.cz>]

> 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
-- 
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