[Xen-devel] More on kexec/purgatory handover

[Jan Beulich]

>>> On 13.05.15 at 11:53, <david.vrabel at citrix.com> wrote:
> 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.

Okay, if the tools do this in v2, then I think the compatibility v1
path should indeed do so too (in the hypervisor).

Jan