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SubjectRe: I/O permissions for V86 tasks (Enhanced V86 Mode)?
> On 	Wed, 15 Jul 1998 11:03:32 -0800, Kendall Bennett said:
> > Actually I have tried compiled the 2.0.35 kernel sources with an 8Kb
> > I/O bitmap (I just grabbed the latest 2.1 sources so we will start
> > hacking on this), and it did not work.
>
> Forgive me if you've already done this but you also need to modify the
> macro which sets TSS descriptors. In 2.0.33 this is the set_tss_desc
> macro in asm/system.h. In my experimental VBE driver I have the following:

No, I haven't done this so this is probably what I am missing. So far
our VBE code works like a champ on most cards I have tested (VBE 1.2
and VBE 2.0) except for ATI's which have timing sensitive code.

> /* The following hack allows the IO_BITMAP to be a nonstandard size */
> #undef set_tss_desc /* asm/system.h */
> #define _set_tss_desc(n,addr,sz) _set_tssldt_desc(((char *) (n)),((int)(addr)),107+sz*4,"0x89")
> #define set_tss_desc(n,addr) _set_tss_desc(n,addr,IO_BITMAP_SIZE)

Cool! I will definately try this out!!

> I have attempted to allow the bitmap to be extended just for the V86 task
> but I don't know whether it works as I haven't tested it since my video
> card just uses the normal VGA I/O ports.

We if you can show me a patch that does this, I can easily try it out
on my end.

> And by the way, the answer to your previous question about IOPL is no,
> the Intel processor does not allow the V86 task to take notice of any IOPL
> so you are forced to do it via the bitmap.

Actually this brings up another issue. I recall reading in Dr Dobb's
(I think) about the undocumented Pentium Enhanced V86 mode. One of
the features of this if I remember correctly is that you can enable
IOPL priveledges for V86 tasks rather than having to use a full 8Kb
I/O bitmap.

Does anyone know any details of this, and if this has been looked
into for the V86 mode support in Linux? I am going to hunt through my
old issues to see if I can find the articles in question, and if the
info is enough I will try to hack the support for this into my kernel
sources. This would be the ideal solution if it would work, since
then we don't have to do anything special to get full I/O access for
V86 tasks (and I am sure this will make the DOSEMU crowd happier).

Regards,


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