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    SubjectRe: USB mass storage and ARM cache coherency
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    On Sun, 2010-02-28 at 05:01 +0000, James Bottomley wrote:
    > On Sun, 2010-02-28 at 11:14 +1100, Benjamin Herrenschmidt wrote:
    > > On Fri, 2010-02-26 at 21:00 +0000, Russell King - ARM Linux wrote:
    > > > On Fri, Feb 26, 2010 at 04:25:21PM +0000, Catalin Marinas wrote:
    > > > > For mmap'ed pages (and present in the page cache), is it guaranteed that
    > > > > the HCD driver won't write to it once it has been mapped into user
    > > > > space? If that's the case, it may solve the problem by just reversing
    > > > > the meaning of PG_arch_1 on ARM and assume that a newly allocated page
    > > > > has dirty D-cache by default.
    > > >
    > > > I guess we could also set PG_arch_1 in the DMA API as well, to avoid the
    > > > unnecessary D cache flushing when clean pages get mapped into userspace.
    > >
    > > That's an interesting thought for us too. When doing I$/D$ coherency, we
    > > have to fist flush the D$ and then invalidate the I$. If we could keep
    > > track of D$ and I$ separately, we could avoid the first step in many
    > > cases, including the DMA API trick you mentioned.
    > >
    > > I wonder if it's time to get a PG_arch_2 :-)
    >
    > Sorry to be a bit late to the party (on holiday), but I/D coherency is
    > supposed to be taken care of using flush_cache_page in the memory
    > mapping routines. On parisc, at least, we don't use any PG_arch flags
    > to help. The way it's supposed to work is that I is invalidated on
    > mapping or remapping, so the I/O code only needs to worry about flushing
    > D. The guarantee we pass to userland is that any page we do I/O to has
    > a clean D cache before it goes back to userspace. Thus if userspace
    > executes the page, the I cache gets its first movein there. There is an
    > underlying assumption to all of this: The CPU won't speculatively move
    > in I cache until the page is executed, so we can rely on the
    > flush_cache_page in the mapping to keep the I cache invalidated until
    > we're ready to execute.

    We cannot guarantee this assumption on ARM. As soon as the page is
    accessible and executable, the CPU can fetch into the I-cache
    speculatively. Even if the page hasn't been mapped into user-space yet,
    we still have the kernel linear mapping via which we can get the same
    I-cache lines fetched (PIPT cache).

    The only place we can safely invalidate the I-cache is after the D-cache
    was flushed (after flush_dcache_page).

    On ARM PIPT, flush_cache_page is a no-op.

    > The other fundamental assumption is that if
    > userspace needs to modify an executable region (say for dynamic linking)
    > it has to take care of reinvalidating the I cache itself ... although it
    > can do this by remapping the region to alter the flags (i.e W no X then
    > X no W).

    The ARM dynamic linker remaps the page with no-exec, writes the data and
    then remaps it back with exec. The COW code flushes the D-cache. Anyway,
    recent dynamic linker no longer touches a code page.
    >
    > But the point of all of this is that I cache invalidation doesn't appear
    > anywhere in the I/O path ... so if we're getting I/D incoherency,
    > there's some problem in the mm code (or there's a missing arch
    > assumption ... like I cache gets moved in more aggressively than we
    > expect). Parisc is very sensitive to I/D incoherency, so we'd notice if
    > there were a serious generic problem here.

    On ARM PIPT, it's probably because flush_cache_page isn't implemented.
    But as I said above, given the speculative fetches I don't think it
    would help much (well, it would work a bit better but not a complete
    fix).

    Thanks.

    --
    Catalin



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