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Re: [Xen-devel] [PATCH RFC 0/7] xen: Clean-up of mem_event subsystem

On Mon, Nov 17, 2014 at 5:37 PM, Jan Beulich <JBeulich@xxxxxxxx> wrote:
>>> On 12.11.14 at 16:48, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 12/11/14 15:31, Tamas K Lengyel wrote:
>>  xen/include/public/domctl.h         |  44 +--
>>  xen/include/public/hvm/params.h     |   2 +-
>>  xen/include/public/mem_event.h      | 134 -------
>>  xen/include/public/memory.h         |   6 +-
>>  xen/include/public/vm_event.h       | 179 +++++++++
> While in principle I think this series is a very good thing, there is a
> problem with editing the pubic header files.
> The contents of mem_event.h is not currently hidden behind #ifdef
> As a result, it is strictly speaking part of the VM-visible public
> API/ABI and not permitted to change in a backwards incompatible manor.
> Having said that, it is currently only usable by privileged domains, so
> there is an argument to be made for declaring that it should have been
> hidden behind __XEN_TOOLS__ in the first place, making it permittable to
> change.

I'm not sure I agree - the meaning of "tools" here would seem quite a
bit different than e.g. in domctl.h. Looking at patch 1, I can't see how
an old consumer (remember that for many of these we have at best
fake consumers in the tree) would deal with the now differently
arranged data. I don't see any versioning of the interface, and hence
I can't see how tools would know which of the formats to expect.


The lack of versioning is a real concern which I have aired during the 4.5 development process. For example, when we switched from HVMEM_access_* to XENMEM_access_* a customer had to do a bunch of manual configure checks to determine what is supported and what isn't. Furthermore, many of the related APIs have changed quite radically between Xen 4.1-4.5, some being abandoned midway just to resurface later in a different context. Going forward having a clear VM_EVENT_VERSION definition would be very helpful and would be the cleanest solution IMHO.


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