Re: Kernel virtual memory?

Rogier Wolff (R.E.Wolff@BitWizard.nl)
Fri, 8 Aug 1997 00:01:41 +0200 (MET DST)

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.

The problem is that the only thing you can't do (except for on a
Sparc (#) ) is solve DMA problems.

> 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.

Yeah. Another part of the kernel might still be having a "physical"
pointer to that memory because some other device is going to DMA
into that memory.

> So, is this possible/feasible/too ugly to implement?

Yeah. Too ugly.

Roger.

(#) Many sparcs use the SBUS which has "DVMA", Direct Virtual Memory
Access: They go through the MMU for DMA accesses.......