RE: [Xen-devel] xen_phys_start for 32b

["Cihula, Joseph" <joseph.cihula@xxxxxxxxx>]

> From: Keir Fraser [mailto:keir.fraser@xxxxxxxxxxxxx]
> Sent: Wednesday, January 07, 2009 7:41 AM
>
> On 07/01/2009 15:13, "Cihula, Joseph" <joseph.cihula@xxxxxxxxx> wrote:
>
> >>> But shouldn't [xenheap_phys_start, xenheap_phys_end] represent all of the
> >>> memory that the hypervisor "owns" and which must be protected from even
> >>> privileged domain writes (modulo the real mode/trampoline code, which has
> >>> its
> >>> own variables that represent its range)?  While it may be "OK" on 32b
> >>> systems,
> >>> it is not "logically correct" and does not match 64b systems (where this 
> >>> low
> >>> memory is not so protected).  Would it break anything to set
> >>> xenheap_phys_start to __pa(&_start) for 32b builds?
> >>
> >> So what issue does this fix for you?
> >
> > It moves the '#ifdef __x86_64__' in a couple of places in an upcoming patch
> > into just setup.c ;-)  So practically speaking, it is not very important.  
> > But
> > it seems like it would just be cleaner, today, to have this variable (and
> > xen_phys_start?) be consistent across builds; and thus, usable with the
> > intended meaning in the future.
>
> Xenheap will disappear entirely on x86/64 in future. So long term is that
> i386 and x86/64 are actually to diverge significantly in this area.
>
> Of course I'll consider any patch on its own merits of usefulness and
> cleanliness.

OK, here is a very small and simple patch to "fix" this.  Note that I used a 
new '#idef' instead of '#else' to the previous because this statement is 
logically distinct from the previous block and there is no difference in the 
generated code.

----------------

For IA32 builds, set xen_phys_start (and by extension, xenheap_phys_start) to 
be the physical address of the start of xen (instead of the previous value, 0).

Signed-off-by: Joseph Cihula <joseph.cihula@xxxxxxxxx>

diff -r e2f36d066b7b xen/arch/x86/setup.c
--- a/xen/arch/x86/setup.c      Mon Dec 22 13:48:40 2008 +0000
+++ b/xen/arch/x86/setup.c      Wed Jan 07 09:19:58 2009 -0800
@@ -868,6 +868,9 @@ void __init __start_xen(unsigned long mb
     nr_pages += (__pa(&_start) - xen_phys_start) >> PAGE_SHIFT;
     vesa_init();
 #endif
+#ifndef __x86_64__
+    xen_phys_start = __pa(&_start);
+#endif
     xenheap_phys_start = xen_phys_start;
     printk("Xen heap: %luMB (%lukB)\n",
            nr_pages >> (20 - PAGE_SHIFT),


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