[coreboot] About Paging, Realmode and what is going on

[ron minnich]

right, Aaron, you keep reminding me of that and I keep forgetting it :-)

so, ok, ramstage it is for paging. The x86 is so darned annoying. We had
paging in the bootblock for PPC on linuxbios.

On Wed, Aug 2, 2017 at 12:43 PM Aaron Durbin <adurbin at google.com> wrote:

> On Wed, Aug 2, 2017 at 1:36 PM, ron minnich <rminnich at gmail.com> wrote:
> >
> >
> > On Wed, Aug 2, 2017 at 12:08 PM Zoran Stojsavljevic
> > <zoran.stojsavljevic at gmail.com> wrote:
> >>
> >>
> >>
> >> So, turning on the Virtual Mode (paging ON), you'll also imply that
> >> Coreboot will initialize and use MMU, don't you?
> >
> >
> > sure.
> >
> >>
> >>
> >> Am I correct, or you can use paging without MMU?!
> >
> >
> > MMU must be on.
> >
> >>
> >> I doubt that, perhaps. So, then, I would like to see this code in
> Coreboot
> >> where MMU is initialized. And how it is initialized (maybe there is a
> some
> >> over-simplified mode of operation using 2MB pages).
> >
> >
> > yep.
> >
> >>
> >>
> >> And my question is: what for? Or I did not get the idea... Who really
> >> needs to use paging in boot-loaders? Even INTEL, which (on purpose)
> makes
> >> things way over-dimensioned and over-complicated, does NOT use paging in
> >> UEFI BIOSes, so far???
> >>
> >
> > it's trivial to turn on paging. In harvey I've got it down to 160 lines,
> > including comments and white space. I made it much more compact in my CL
> > from 1/2014.
> >
> > If you turn on paging you can capture null pointer derefs, and all other
> out
> > of range address references. And relocatable modules are much easier..
> >
> > My question is, why not do it? I don't see any harm. All the other
> platforms
> > in coreboot use paging. What's so special about x86 that it should not
> use
> > paging?
>
> You can't use it for CAR so that sorta throws out romstage and before.
> It's up to the implementation of how CAR works within the hierarchy,
> but with hardware page walkers that pull things through the cache
> things can go badly. So then you could enable paging for ramstage.
> Need to locate storage for the page tables, but that's not an
> impossible thing either.
>
> >
> > ron
> >
> > --
> > coreboot mailing list: coreboot at coreboot.org
> > https://mail.coreboot.org/mailman/listinfo/coreboot
>
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