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SubjectRe: Missing cache flush.


On Tue, 5 Jun 2001, David Woodhouse wrote:

>
> davem@redhat.com said:
> > Chris Wedgwood writes:
> > > What if the memory is erased underneath the CPU being aware of
> > > this? In such a way ig generates to bus traffic...
>
> > This doesn't happen on x86. The processor snoops all transactions
> > done by other agents to/from main memory. The processor caches are
> > always up to date.
>
> No. My original mail presented two situations in which this assumption is
> false.
>
> 1. Bank-switched RAM. You want to force a writeback before switching pages.
> I _guarantee_ you that the CPU isn't snooping my access to the I/O port
> which sets the latch that drives the upper few address bits of the RAM
> chips.
>
> 2. Flash. A few writes of magic data to magic addresses and a whole erase
> block suddenly contains 0xFF. The CPU doesn't notice that either.

3. Buggy implementations like the Cyrix 486es that don't properly maintain
cache coherency.

kira.

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