Re: 2.6.25-rc1 xen pvops regression

[H. Peter Anvin]

Jeremy Fitzhardinge wrote:
> > What is the e820 information you feed the kernel?  We should only ever 
> > allocate page tables out of available RAM, not any other type of 
> > memory (reserved or not). 
>
> The kernel gets a flat memory map; all memory is just plain RAM.  The 
> problem is that we're allocating a normal page and turning it into a 
> pagetable - so far so good.  Then the DMI code is randomly mapping that 
> same page RW so it can scan it for DMI signatures, which Xen is preventing.
>
> There are two immediate fixes:
>
>   1. Only scan for DMI if the memory is reserved (rejected, because HPA
>      says some machines don't reserve the DMI space).  Alternatively,
>      don't bother scanning if booting under Xen.
>   2. Make DMI map the memory RO so that Xen doesn't complain (which is
>      sensible because DMI is ROM anyway).
>
> But as far as I can tell, this shouldn't be happening anyway, and could 
> happen on real hardware which doesn't reserve the DMI space.  It 
> probably doesn't because initial pagetables on real hardware use large 
> pages, and therefore allocate less memory for pagetable memory and 
> therefore doesn't end up hitting the 0xf0000 region.  But that area 
> should be excluded from the allocation pool.

Which it is on real hardware, because although it's not *reserved* (type 
2), it is certainly not made available as *normal memory* (type 1).  If 
Xen maps this as type 1 then I definitely see the problem.

We can exclude type 1 memory from DMI scan, certainly.

However, Xen may want to consider why provide memory below the 1 MB 
point at all, and certainly whether it's wise to provide RAM in the 
640-1024 KB legacy region -- although you could argue that "it *should* 
work", odds are pretty good you'll have nasty surprises on a regular basis.

	-hpa

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