Re: 2.6.25-rc1 xen pvops regression

["H. Peter Anvin" <hpa@zytor.com>]

Jeremy Fitzhardinge wrote:
> Ian Campbell wrote:
>> I'll see if I can track down where the page is getting used and have a
>> go at getting in there first. It must be pretty early to be allocated
>> already when dmi_scan_machine gets called.
>>
>> It's possible that the domain builder might have already allocated a PT
>> at this address. I haven't checked but I think currently the domain
>> builder always puts PT pages after the kernel so hopefully it's only a
>> theoretical problem.
>>   
> 
> Yes, it does.  And presumably the early pagetable builder is guaranteed 
> to avoid special memory like the DMI space.  But the bug definitely 
> seems to be a result of the DMI code trying to make a RW mapping of a 
> pagetable page, so something is amiss there.
> 
> Ooh, sleazy hack idea: make DMI always map RO, so even if it does get a 
> pagetable it causes no complaint...  A bit awkward, since there doesn't 
> seem to be an RO form of early_ioremap.
> 
>> Another option I was thinking of was a command line option to disable
>> DMI, which (maybe) isn't terribly useful in itself but it introduces an
>> associated variable to frob with. That's similar to how the TSC was
>> handled in the past (well, the opposite since TSC was forced on).
>>   
> 
> Yep, that would work too.
> 
> Still curious about why a pagetable page is ending up in that range 
> though.  Seems like it shouldn't be possible, since we shouldn't be 
> allowed to allocate from those pages, at least until the DMI probe has 
> happened...  Unless the early allocator is only excluded from e820 
> reserved pages, which would cause a problem on systems which don't 
> reserve the DMI space...  HPA?
> 

I thought the problem was a Xen-provided pagetable from before Linux 
started?

	-hpa