[Xen-devel] [PATCH 3/8] kexec: add public interface for improved load/unload sub-ops

[Jan Beulich]

>>> On 08.03.13 at 13:28, Daniel Kiper <daniel.kiper at oracle.com> wrote:
> On Fri, Mar 08, 2013 at 11:52:21AM +0000, David Vrabel wrote:
>> On 08/03/13 10:50, Daniel Kiper wrote:
>> > On Thu, Feb 21, 2013 at 05:48:09PM +0000, David Vrabel wrote:
>> >> From: David Vrabel <david.vrabel at citrix.com>
>> >>
>> >> Add replacement KEXEC_CMD_load and KEXEC_CMD_unload sub-ops to the
>> >> kexec hypercall.  These new sub-ops allow a priviledged guest to
>> >> provide the image data to be loaded into Xen memory or the crash
>> >> region instead of guests loading the image data themselves and
>> >> providing the relocation code and metadata.
>> >>
>> >> The old interface is provided to guests requesting an interface
>> >> version prior to 4.3.
>> >>
>> >> Signed-off: David Vrabel <david.vrabel at citrix.com>
>> >
>> > [...]
>> >
>> >> diff --git a/xen/include/public/kexec.h b/xen/include/public/kexec.h
>> >> index 61a8d7d..5259446 100644
>> >> --- a/xen/include/public/kexec.h
>> >> +++ b/xen/include/public/kexec.h
>> >> @@ -116,12 +116,12 @@ typedef struct xen_kexec_exec {
>> >>   * type  == KEXEC_TYPE_DEFAULT or KEXEC_TYPE_CRASH [in]
>> >>   * image == relocation information for kexec (ignored for unload) [in]
>> >>   */
>> >> -#define KEXEC_CMD_kexec_load            1
>> >> -#define KEXEC_CMD_kexec_unload          2
>> >> -typedef struct xen_kexec_load {
>> >> +#define KEXEC_CMD_kexec_load_v1         1 /* obsolete since 0x00040300 */
>> >> +#define KEXEC_CMD_kexec_unload_v1       2 /* obsolete since 0x00040300 */
>> >> +typedef struct xen_kexec_load_v1 {
>> >>      int type;
>> >>      xen_kexec_image_t image;
>> >> -} xen_kexec_load_t;
>> >> +} xen_kexec_load_v1_t;
>> >
>> > I think that this is not good idea to redefine meaning of constants,
>> > types, structures, etc. IMO it is comparable to redefining meaning
>> > of words in any laguage (e.g. English). It will be very confusing
>> > and may easily lead to stupid bugs. I think that old interface should
>> > stay as is (with its bad behavior). New interface should be introduced
>> > with "_v2" suffix, e.g. KEXEC_CMD_kexec_load_v2, ...
>> > This would not confuse our descendants.
>>
>> This is something that was requested (by Ian C) as the Xen way of doing it.
> 
> Yes, I remember but still do not agree with that idea in general.
> Maybe discussion on kexec interface is good point to change
> that Xen community behavior? Ian?

Together with all consumers being expected to properly make
use of __XEN_INTERFACE_VERSION__ (or else they get the
lowest possible definitions), I don't think there's a big issue here
as long as all old definitions retain their meaning when said
symbol is set low enough.

Jan