From mboxrd@z Thu Jan 1 00:00:00 1970 From: Andrew Cooper Subject: Re: Xen-4.5 HVMOP ABI issues Date: Thu, 4 Dec 2014 15:46:30 +0000 Message-ID: <548081D6.4080208@citrix.com> References: <54785E46.4080107@citrix.com> <5478819F020000780004B70D@mail.emea.novell.com> <54787ED8.2010508@citrix.com> <5478A056020000780004B7CE@mail.emea.novell.com> <547898E7.30400@citrix.com> <5480748B020000780004CBFC@mail.emea.novell.com> <54806FA2.6080406@citrix.com> <548083EE020000780004CC90@mail.emea.novell.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <548083EE020000780004CC90@mail.emea.novell.com> List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xen.org Errors-To: xen-devel-bounces@lists.xen.org To: Jan Beulich , Konrad Rzeszutek Wilk Cc: Tim Deegan , IanCampbell , Xen-develList List-Id: xen-devel@lists.xenproject.org On 04/12/14 14:55, Jan Beulich wrote: >>>> On 04.12.14 at 15:28, wrote: >> On 04/12/14 13:49, Jan Beulich wrote: >>>>>> On 28.11.14 at 16:46, wrote: >>>> On 28/11/14 15:18, Jan Beulich wrote: >>>>>>>> On 28.11.14 at 14:55, wrote: >>>>>> The problem is with continuations which reuse the upper bits of the >>>>>> input register, not with this HVMOP_op_mask specifically; the >>>>>> HVMOP_op_mask simply adds to an existing problem. This is something >>>>>> which needs considering urgently because, as you identify above, we have >>>>>> already suffered an accidental ABI breakage with the mem-op widening. >>>>> Since we can't retroactively fix the mem-op widening, I still don't see >>>>> what's urgent here: As long as we don't change any of these masks, >>>>> nothing bad is going to happen. Of course one thing to consider with >>>>> this aspect in mind is whether to change the hvm-op or gnttab-op >>>>> masks again _before_ 4.5 goes out, based on whether we feel they're >>>>> wide enough for the (un)foreseeable future. >>>> By urgent, I mean exactly this, while we have the ability to tweak the >>>> masks. >>> With no-one else voicing an opinion: >>> >>> For hvmop, the mask currently is 8 bits and we've got 22 ops defined. >>> >>> For gnttabop, the mask currently is 12 bits and we've got 12 ops defined. >>> >>> For the latter, we're fine even without further consideration. For the >>> former, the two operations actively using the continuation encoding >>> are tools-only ones. Since we're fine to alter the tools only interfaces, >>> and since it was intended for the tools-only HVM-ops to be split off >>> to a separate hypercall (e.g. hvmctl) anyway, the range restriction >>> would then no longer be a problem. Plus, in the worst case we could >>> always introduce yet another hypercall if we ran out of numbers. >> Are you suggesting that we make a new hvmctl now and remove the hvmop >> mask before 4.5? If we ship 4.5 with the hvmop mask, we cannot >> subsequently remove it even if all continuable hypercalls move to a >> separate hypercall. > Why? We certainly don't guarantee compatibility for undefined > hypercalls to behave in a certain way. A task is in the middle of a hypercall continuation. The VM is migrated to a newer Xen which has lost the op mask and gained a new hypercall which would alias. At this point, the task has transparently swapped hypercall subops, through no fault of its own. As I have said before, once an op mask has found its way into a released version of Xen, it is irrevocably part of the ABI, and cannot change in the future. ~Andrew