Re: Xen 4.0 crashes with pvops kernel

["Jan Beulich" <JBeulich@novell.com>]

>>> On 15.06.10 at 15:20, "Jan Beulich" <JBeulich@novell.com> wrote:
>>>> On 15.06.10 at 14:56, Keir Fraser <keir.fraser@eu.citrix.com> wrote:
>> On 15/06/2010 13:50, "Jan Beulich" <JBeulich@novell.com> wrote:
>> 
>>> But even if we verify that the references come from some ACPI
>>> method(s), likely the only way to address this is to fix the kernel
>>> side error handling.
>>> 
>>> Keir, assuming these are reads only, would it make sense to permit
>>> Dom0 to map the IO-APIC space read-only? Perhaps even
>>> transparently converting writeable mappings to read-only ones
>>> (since drivers/acpi/osl.c tries to establish writeable mappings
>>> irrespective of the actual needs)? The obvious danger in doing
>>> so is that going forward there may appear fields in that page
>>> reads of which aren't side effect free...
>> 
>> Well, how come it works with other Linux kernels -- presumably they have
>> some extra error handling in the ACPI subsystem? Shouldn't that just be
>> added to this kernel?
> 
> I'm rather suspecting there's new code (compared to 2.6.18) that's
> lacking proper error handling, though I didn't look in detail so far.
> 
> Hmm, looking a little more closely it seems they indeed try to write
> to that space - this we for sure can't allow. I'll see if I can follow
> the code path (unfortunately the stack trace is an imprecise one).

Actually, that's a difference to non-pv-ops that I strongly
believe should be fixed: While in the traditional kernel
__direct_remap_pfn_range() is used to establish I/O memory
mappings (and hence there is a way to propagate errors), the
pv-ops kernel appears to use ioremap_page_range() - just like
native - which can only return -ENOMEM (upon page table
allocation failure), due to the lack of a return value from
set_pte_at().

But then again I must be missing something here, since
xen_set_pte_at() falls back to xen_set_pte() if the hypercall
it tries first fails, and that one would fault when establishing
the mapping, not when trying to first use it. Jeremy?

Jan