[Xen-devel] Re: [PATCH 3/8] xen/setup: Set identity mapping for non-RAM E820 and E820 gaps.

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

> > For the privileged guest - yes. But for the non-priviligied it does not
> > have such range and would end up failing.
> 
> xen_memory_setup has:
>         e820_add_region(ISA_START_ADDRESS, ISA_END_ADDRESS -
> ISA_START_ADDRESS, E820_RESERVED);
> which is unconditional but is actually more for domU's benefit than
> dom0's which already sees the host e820 presumably with the right hole
> already in place, which we simply shadow, or maybe slightly extend,
> here.

Actually we don't do anything with that region in Dom0 case. We just
return the PFN without consulting the P2M for 0->0x100 while for DomU _we_
do consult the P2M and set those in the PTE. (look in xen_make_pte)

> 
> In a domU we do this because if you let these pages into the general
> allocation pool then they will potentially get used as page table pages
> (hence be R/O) but e.g. the DMI code tries to map them to probe for
> signatures and tries to does so R/W which fails. We could try and find
> everywhere in the kernel which does this or we can simply reserve the
> region which stops it getting used for page tables or other special
> things, and is somewhat less surprising for non-Xen code.

Yeah, went that hole once.. too many generic pieces of code.
.. snip..
> > You mean the ISA_START_ADDRESS->ISA_END_ADDRESS we mark as reserved?
> 
> Yep.
> 
> > It sure would be easier
> > 
> > (and it would mean we can return that memory back to the hypervisor).
> 
> I don't think you can return it, since something like the DMI code which
> wants to probe it expects to be able to map that PFN, if you've given
> the MFN back then that will fail.

Correct (for non-priviliged PV domain).
> 
> I suppose we could alias all such PFNs to the same scratch MFN but I'd

It actually works. I setup 0x1->0x100 to point to whatever the MFN was at
0x0, and released the pages from 0x1->0x100 and it worked for DomU PV guests 
(and dom0 since I ended up stomping those regions with the PFN|
IDENTITY_BIT_FRAME).

However, the tools weren't happy ('xm save'). They did not like the same PFN 
across a couple of entries in the P2M table and complained about a potential 
race. But there is another way and that is to special case in 'xen_make_pte' 
when we want to create a PTE for 0->ISA_END_ADDRESS and just give it the MFN 
from P2M[0x0] (for !xen_initial_domain()) while having the the pfns from 0x1-
>0x100 freed and set to be IDENTITY_BIT_FRAME... But that all just smacks of 
weird corner cases. Thought the code that is there is already special casing 
access to that region. Maybe it would clear it up a bit.


> be concerned about some piece of code which expects to interact with
> firmware scribbling over it and surprising some other piece of code
> which interacts with the firmware...

Fortunatly the all look for a some signature first before trying to scribble.

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