PPC/MCA RS/6000 port ?

[Dave Weis]

On Tue, 21 Mar 2000, Gabriel Paubert wrote:
> On Tue, 21 Mar 2000, Dave Weis wrote:
> > On Mon, 20 Mar 2000, Gabriel Paubert wrote:
> > > However, if the use direct store segments is _unavoidable_, significant
> > > kernel modifications are required.
> > I know we've (Gabriel and I) discussed this, but could you elaborate?
> > Based on what I've read in my docs, I think it is unavoidable. I'm getting
> > closer to having the boot process figured out. Once I've got some more
> > questions, I'm going to send my info to the list and see if my data looks
> > accurate.
>
> Right now the segment registers are only changed on task switches.
> To access devices which are addressed through direct store segments, you
> have to load one segment register (which occupies 256Mb of virtual address
> space) when you enter the driver for this device.

The way the drivers/info I've seen do it is to call iomem_att when they
need access to whatever they are looking for. I had a tarball of aix
driver source for a few devices but can't seem to find it now. If anyone
else knows where to find it, please let me know. The docs on this function
are viewable at
http://aix43.uwaterloo.ca/aix/doc_link/en_US/a_doc_lib/libs/ktechrf1/iomem_att.htm

> You might also have to save/restore the segment register on interrupts,
> and you'd better remove this segment when returning to user space or you
> may create a huge security hole.

True.

> The best solution might be to designate one segment as reserved for this
> purpose (say the last one) and use a save/restore mechanism in each
> routine which needs to acces this kind of device. You have to make sure
> that all paths restore it to the state it had on entry, but at least you
> don't have to muck with the low level system call and interrupt entry and
> exit code.

I think the aix way would probably work okay, especially since there's a
minimum number of drivers that would work on the machine anyway.

dave

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