Re: [Xen-devel] L1[0x1fb] = 0000000000000000 which faults on one type of machine but on another works?

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Thu, Mar 17, 2011 at 10:25:11AM +0000, Jan Beulich wrote:
> >>> On 16.03.11 at 23:19, Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx> 
> >>> wrote:
> > But one thing I can't understand is why on one machine (IBM x3850)
> > I get this crash, while another one with the same pagetable contents
> > (L1 has nothing for 0x1fb) it works just fine? I added a panic and used
> > the Xen hypervisor kdb to manually inspect the pagetable, and it has
> > the same contents as the IBM x3850 -but it boots fine with this invalid 
> > value.
> > Any ideas?
> 
> Without seeing the full stack trace it's hard to tell. To me, it looks
> like a mistake for native_apic_read() to be called at all under Xen,
> and perhaps there's one lurking somewhere that gets hit only on
> those IBM (Summit?) machines.

That was it. When we bootup we call 'set_xen_basic_apic_ops' which
sets apic->read to xen_apic_read. The default 'apic' is set to
apic_flat, so in essence we change apic_flat->read from native_apic_read
to xen_apic_read.

During bootup, the default_acpi_madt_oem_check is run which
runs through all of the apic_probe[] array, on which the last
one is is apic_physflat. And apic_physflat->probe() returns true
on this IBM Summit box (and ES7000 boxs, and whatever has FADT
set to ACPI_FADT_APIC_PHYSICAL) so we set apic now to apic_physflat
and the apic->read ends up being native_apic_read.

2.6.38 fixes this by allowing in acpi_register_lapic_address, the
the set_fixmap_nocache(FIX_APIC_BASE, address) to be called and we
can provide it with a dummy page and native_apic_read can happily
read from that fake page.



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