Re: X server in dom0: Bad VBT signature

[Manuel Bouyer <bouyer%antioche.eu.org@localhost>]

On Sun, Sep 02, 2012 at 08:37:34AM +0100, Cherry G. Mathew wrote:
> Index: arch/xen/include/xenfunc.h
> ===================================================================
> RCS file: /cvsroot/src/sys/arch/xen/include/xenfunc.h,v
> retrieving revision 1.15
> diff -u -r1.15 xenfunc.h
> --- arch/xen/include/xenfunc.h        23 Oct 2009 02:32:33 -0000      1.15
> +++ arch/xen/include/xenfunc.h        2 Sep 2012 07:32:24 -0000
> @@ -43,4 +43,17 @@
>  #endif
> 
>  void xen_set_ldt(vaddr_t, uint32_t);
> +
> +#if defined(DOM0OPS)
> +
> +/* This is part of a hack to allow dom0 to access isa i/o "hole"s */
> +static __inline bool
> +xen_dom0_pa_isa_conflict(paddr_t pa)
> +{
> +     extern paddr_t pmap_pa_end;
> +     return __predict_false(pa < 0x100000 || pa >= pmap_pa_end);
> +}
> +
> +#endif /* DOM0OPS */
> +
>  #endif /* _XEN_XENFUNC_H_ */
> Index: arch/x86/x86/pmap.c
> ===================================================================
> RCS file: /cvsroot/src/sys/arch/x86/x86/pmap.c,v
> retrieving revision 1.178
> diff -u -r1.178 pmap.c
> --- arch/x86/x86/pmap.c       15 Jun 2012 13:53:40 -0000      1.178
> +++ arch/x86/x86/pmap.c       2 Sep 2012 07:32:29 -0000
> @@ -985,7 +985,12 @@
>       else
>               pte = kvtopte(va);
>  #ifdef DOM0OPS
> -     if (pa < pmap_pa_start || pa >= pmap_pa_end) {
> +     if (xen_dom0_pa_isa_conflict(pa)) { /* dom0 wants to access ISA mem,
> not ram */
> +             /*
> +              * This is a hack. The right thing to do would be to
> +              * have bus_space_map() et al. manage the p2m/m2p
> +              * tables.

I don't understand what you mean here; bus_space_map() already properly
calls pmap_enter_ma() with a machine address.
This problem only exists for userland accessing devices through /dev/mem,
because we want /dev/mem to map both physical or machine addresses,
depending on what the caller means. The right fix would be to have a
way to express the address space at this level. But this is intrusive.

Another problem with your patch is that you don't prevent the memory in
range pa < 0x100000 from being used by UVM. So you'll end up with
regular pages mapped in device's memory space, which will likely lead to
panic or spontaneous reboot.


-- 
Manuel Bouyer <bouyer%antioche.eu.org@localhost>
     NetBSD: 26 ans d'experience feront toujours la difference
--