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Before the >>>>>>> VM starts running, we limit the feature set to the subset we actually >>>>>>> support with NV. >>>>>>> >>>>>>> By doing this, we also change the value of IDreg fields that are not >>>>>>> writable, because they describe features that we don't support. >>>>>>> Obviously, that fails on restore. >>>>>>> >>>>>>> I need to have a think... >>>>>> >>>>>> We spoke about this a while ago (and I forgot til now), but I was >>>>>> wondering if we could use vCPU feature flags to describe NV, including >>>>>> the selection between FEAT_E2H0 and FEAT_VHE. >>>>>> >>>>>> I think this might match userspace expectations a bit more closely where >>>>>> the state of the ID registers after init gives the actual feature set >>>>>> supported by the VM. >>>>> >>>>> I'm not sure that's enough. Let me give you an example: >>>>> >>>>> My host has FEAT_XNX, described in ID_AA64MMFR1_EL1.XNX. For whatever >>>>> reason, we don't allow this field to be written to, even out of NV >>>>> context. This is odd, because for an EL1 VM, this field means nothing >>>>> at all. >>>> So the curprit fields for me look like >>>> >>>> - ID_AA64MMFR1_EL1.XNX >>>> - ID_AA64DFR0_EL1.DoubleLock >>>> - ID_AA64PFR0_EL1.RAS >>>> >>>> This is still based on your nv-next branch from Jan 9 >>>> https://github.com/eauger/linux/tree/nv_next_jan9_2025 >>> >>> I have now pushed out a new nv-next branch with the new and improved >>> UAPI. I expect migration to work a bit better, or at least not to >>> explode on ID register restore. You will notice that things have >>> changed a bit (extra flag and cap for FEAT_E2H0), but nothing really >>> major. >>> >> >> Tried nv-next branch and it is breaking(kernel Oops) for normal VM >> boot itself with qemu. Looks like this is happening since qemu is >> trying to write to ID_UNALLOCATED mapped registers as part of >> save-restore of registers. > > My take on this problem ends up being more consolidation, and make > sure that the individual macros only override the default callbacks > for idregs. > > Additionally, ID_UNALLOCATED gets a name matching the architectural > encoding. > Thanks, having name helps future debugging. While I was debugging, the register name was showing as null and I have to map id to encodings. I have rebased to nv-next and looks all good!. -- Thanks, Ganapat/GK