Re: [Xen-devel] [PATCH] xen: find a better location for the real-mode trampoline

[Konrad Rzeszutek Wilk <konrad@xxxxxxxxxx>]

On Fri, Nov 30, 2012 at 08:33:34AM +0000, Jan Beulich wrote:
> >>> On 29.11.12 at 18:34, Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote:
> > On some machines, the location at 0x40e does not point to the beginning
> > of the EBDA.  Rather, it points to the beginning of the BIOS-reserved
> > area of the EBDA, while the option ROMs place their data below that
> > segment.
> > 
> > For this reason, 0x413 is actually a better source than 0x40e to get
> > the location of the real-mode trampoline.  But it is even better to
> > fetch the information from the multiboot structure, where the boot
> > loader has placed the data for us already.
> 
> I think if anything we really should make this a minimum calculation
> of all three (sanity checked) values, rather than throwing the other
> sources out. It's just not certain enough that we can trust all
> multiboot implementations.
> 
> Of course, ideally we'd consult the memory map, but the E820 one
> is unavailable at that point (and getting at it would create a
> chicken-and-egg problem).

Can we scan the memory for the possible EBDA regions? There is an
"EBDA" type header in those regions, if I recall?
> 
> Jan
> 
> > Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> > ---
> >  xen/arch/x86/boot/head.S | 21 ++++++++++++---------
> >  1 file changed, 12 insertions(+), 9 deletions(-)
> > 
> > diff --git a/xen/arch/x86/boot/head.S b/xen/arch/x86/boot/head.S
> > index 7efa155..1790462 100644
> > --- a/xen/arch/x86/boot/head.S
> > +++ b/xen/arch/x86/boot/head.S
> > @@ -78,16 +78,19 @@ __start:
> >          cmp     $0x2BADB002,%eax
> >          jne     not_multiboot
> >  
> > -        /* Set up trampoline segment 64k below EBDA */
> > -        movzwl  0x40e,%eax          /* EBDA segment */
> > -        cmp     $0xa000,%eax        /* sanity check (high) */
> > -        jae     0f
> > -        cmp     $0x4000,%eax        /* sanity check (low) */
> > -        jae     1f
> > -0:
> > -        movzwl  0x413,%eax          /* use base memory size on failure */
> > -        shl     $10-4,%eax
> > +        /* Set up trampoline segment just below end of base memory.
> > +         * Prefer to get this information from the multiboot
> > +         * structure, if available.
> > +         */
> > +        mov     4(%ebx),%eax        /* kb of low memory */
> > +        testb   $1,(%ebx)           /* test MBI_MEMLIMITS */
> > +        jnz     1f
> > +
> > +        movzwl  0x413,%eax          /* base memory size in kb */
> >  1:
> > +        shl     $10-4,%eax          /* convert to a segment number */
> > +
> > +        /* Reserve 64kb for the trampoline */
> >          sub     $0x1000,%eax
> >  
> >          /* From arch/x86/smpboot.c: start_eip had better be page-aligned! 
> > */
> > -- 
> > 1.8.0
> > 
> > 
> > _______________________________________________
> > Xen-devel mailing list
> > Xen-devel@xxxxxxxxxxxxx 
> > http://lists.xen.org/xen-devel 
> 
> 
> 
> 
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> 

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