On 10/04/17 16:17, Auger Eric wrote: > Hi Marc, > > On 10/04/2017 16:57, Marc Zyngier wrote: >> On 10/04/17 15:32, Auger Eric wrote: >>> Hi Marc, >>> >>> On 08/04/2017 12:42, Marc Zyngier wrote: >>>> On Mon, Mar 27 2017 at 10:30:58 AM, Eric Auger <eric.auger@xxxxxxxxxx> wrote: >>>>> The GITS_IIDR revision field is used to encode the version of the >>>>> table layout (ABI). So we need to restore it to check the table >>>>> layout to be restored is compatible with the destination vITS. >>>>> >>>>> The user selected revision is stored in the user_revision field. >>>>> It will be compared against the REV num at table restoration time. >>>> >>>> Why isn't it sufficient to keep it GITS_IIDR RO and let userspace find >>>> out about the ABI revision that the kernel understands? >>>> >>>> Or are we planning on supporting multiple ABIs in the kernel? >>> Yes as discussed with Peter, the plan is to allow several ABIs. So the >>> userspace informs the destination about the ABI revision of the stored >>> tables (contained by the GITS_IIDR). If the destination KVM does not >>> support this ABI revision, table restore will fail. >>> If so, do >>>> we have a deprecation policy/plan? I don't mind either way, but it would >>>> be good to document it... >>>> >>>> Maybe it is documented already and I missed it (which is perfectly >>>> possible!). >>> Well this is partly documented in >>> Documentation/virtual/kvm/devices/arm-vgic-its.txt. No plan to >>> deprecate. migration from KVM supporting v1 to KVM supporting V2 would >>> be possible but not the contrary. >>> >>> Does it make sense? >> >> Sort of. Say you have three systems: A and C, which only supports v1; B, >> which supports v1 and v2. Let's say you migrate from A to B, and from B >> to C. Is B mandated to be able to export the tables as v1 and v2? Or can >> it restrict what it can export? > At the moment migration from B to C will fail because source ABI rev = > v2 > destination support ABI = v1. > > A (v1) -> B (v1 & v2): migration OK > B (v1 & v2) -> C (v1): migration NOK So what does IIDR report on B once the A->B migration has taken place? Does it report v2? Thanks, M. -- Jazz is not dead. It just smells funny...
