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SubjectRe: Kernel virtual memory?
On Thu, 7 Aug 1997, Bryn Paul Arnold Jones wrote:

> -----BEGIN PGP SIGNED MESSAGE-----
>
> On Thu, 7 Aug 1997, Rauli Ruohonen wrote:
>
> > Could kernel use virtual memory hardware to make non-linear chunks of
> > memory to look linear? It seems to me that it could solve the problems
> > with memory fragmentation and DMA memory allocation.
> >
> > DMA memory could be allocated by moving allocated blocks away from the
> > lower 16MB, and other parts of kernel could be made happy by changing the
> > page tables accordingly.
> >
> > So, is this possible/feasible/too ugly to implement?
> >
>
> Erm DMA stands for direct memory access, the CPU doesn't get a look-in
> (past seting it all up), and so can't mess about (that's the advantage,
> the CPU isn't tied up). The DMAing is actually done by whatever card is
> transferring the data, and so it's talking to physical memory using
> physical addresses.

You *can* get away with that sort of thing, but only if (1) you can
guarantee that no I/O buffer will ever cross a page boundary, (2) you
write all drivers so that they lock down all buffers in physically
contiguous memory before starting an I/O, or (3) all of your peripheral
controllers will accept scatter/gather lists and you're willing to write
drivers that can set them up. (2)'s the way you usually see it, but
that'd require rewriting a lot of drivers if it wasn't designed in from
Day One.

Mark H. Wood, Lead System Programmer MWOOD@INDYVAX.IUPUI.EDU
Those who will not learn from history are doomed to reimplement it.


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