[Xen-devel] More on kexec/purgatory handover

[David Vrabel]

On 13/05/15 09:12, Jan Beulich wrote:
>>>> On 13.05.15 at 09:35, <ebiederm at xmission.com> wrote:
>> Fundamentally if you are transfering control in long mode you have to
>> set up some page table.  I giant identity mapped page table that can use
>> 1G or 2M pages takes up very little memory, and can be very simply
>> and easily before the transfer of control takes place.
>>
>> All you have to do when you are in a half broken state is load cr3.
>> Possible after verifying a checksum.
>>
>> 640k in this case I don't think is particularly special, and certainly
>> not worth a special case.  The in-kernel implementation on x86_64 sets
>> up a page table for all of memory which because of the availability of
>> huge pages winds up being simple and trivial.
>>
>> Weird things like copying off the 640k region for the kexec-on-panic
>> case can be done in the adapter/purgatory piece that lives between the
>> two kernels.
>>
>> So at a very practical level I think we shouldn't have mappings for
>> special regions we should just have mappings for all of memory.
> 
> But in all of the above you (a) forget that setting up 1:1
> mappings for all memory isn't as simple as putting in place a
> couple of 1G pages - holes need to be accounted for and must
> at best be mapped UC (that's especially an issue with the low
> 640k) and (b) imply that whatever Linux behavior there is, Xen
> should mimic it (ignoring for example the fact that with the non-
> kernel based kexec which newer Xen and tools support such 1:1
> mapping setup doesn't appear to be required, i.e. [supposed]
> requirements change).

Xen's V2 kexec ABI builds 1:1 pages tables for the source and
destination pages and any additional regions requested by the guest (see
calls to machine_kexec_add_page()).  kexec-tools adds a "map-only"
segment for 0-1MiB when using the V2 ABI.

These page tables are built at load time (not at exec time) and in the
crash case are placed in the crash memory area.

When using the V1 ABI, there is no way for the tools to provide an
additional "map-only" segment so you'd have to get purgatory to add
mappings for 0-1MiB, or get Xen (in the V1 path only) to do so.

David