On 8 July 2015 at 16:56, Marc Zyngier <marc.zyngier@xxxxxxx> wrote: > On 29/06/15 18:37, Peter Maydell wrote: >> On 29 June 2015 at 18:20, Claudio Fontana <claudio.fontana@xxxxxxxxxx> wrote: >>> On 26.06.2015 06:49, Jan Kiszka wrote: >>>> QEMU has the concept of write-back levels: KVM_PUT_RUNTIME_STATE, >>>> KVM_PUT_RESET_STATE and KVM_PUT_FULL_STATE. I suspect this registers is >>>> just sorted into the wrong category, thus written as part of the >>>> RUNTIME_STATE. We had such bug patterns during the x86 maturing phase as >>>> well. >> >>> It seems that QEMU target-arm ignores the level parameter to >>> kvm_arch_put_registers completely. >>> >>> Is it intended? >> >> Yes, sort of. We don't in general know anything about the semantics >> of most of the system registers. It should always be safe to >> read them all out of the kernel and write them back... > > I'm not sure you can safely assume this for time related things, unless > you can guarantee that all vcpus are stopped. Claudio is seeing time > jumping in weird ways, and so have I, which would tend to show that QEMU > is introducing some jitter. > > Maybe not easily observable on real hardware, but the FastModel is > enough to show the issue. > > So unless someone has a better solution, I'm seriously considering > getting this patch merged. I'd prefer it if somebody could investigate to see why QEMU is actually doing this -- so far we just have speculation. -- PMM -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
