Re: RFC: EFER in crash notes

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 25/09/12 15:33, Ian Campbell wrote:
> On Tue, 2012-09-25 at 15:18 +0100, Andrew Cooper wrote:
>> While this patch is very simple, and I hope without any objection, it is
>> RFC for the reason that our crash ABI is private.
>>
>> The comments for XEN_ELFNOTE_CRASH_REGS does state that it is
>> architecture specific, and makes no indication about the size or
>> contents of the crash note.  However, any code trying to use one of
>> these types of notes has to make an assumption that it if the note desc
>> length is 4*8 bytes long, it is representing CR{0,2-4}.
>>
>> I guess my question boils down to whether it is acceptable to change a
>> private ABI which is not really so private, or whether we should make a
>> formal public ABI for all of the inards of the crash notes and use that.
> Is it really private? (or even "not so private"), don't external tools
> like crash support it?

This is my point.  It is not in xen/public but is used by crash/kdump
and similar utilities, effectively making it a public ABI.

include/public/elfnote.h even explicitly refers to
include/xen/elfcore.h, which is not public by our definition.

>
> I suppose the size field in the notes is a sort of rudimentary version
> field. Remember you can always add a new note type though.

Yes, although looking through my code, I do raise an error if
sizeof(note->desc) != sizeof(my structure representing this note), which
was put in with the best of intentions, but will break with the this RFC
change.

On the other hand, adding a new crash note for every new register will
not scale well, as it is per PCU.

I guess the only sensible way to continue is to present a formal public ABI.

>
> Ian.
>
>
-- 
Andrew Cooper - Dom0 Kernel Engineer, Citrix XenServer
T: +44 (0)1223 225 900, http://www.citrix.com