On Tue, 14 Dec 2021 00:41:01 +0000, Qu Wenruo <quwenruo.btrfs@xxxxxxx> wrote: > > > > On 2021/12/14 00:49, Marc Zyngier wrote: > > On Mon, 13 Dec 2021 16:06:14 +0000, > > Peter Maydell <peter.maydell@xxxxxxxxxx> wrote: > >> > >> KVM on big.little setups is a kernel-level question really; I've > >> cc'd the kvmarm list. > > > > Thanks Peter for throwing us under the big-little bus! ;-) > > > >> > >> On Mon, 13 Dec 2021 at 15:02, Qu Wenruo <quwenruo.btrfs@xxxxxxx> wrote: > >>> > >>> > >>> > >>> On 2021/12/13 21:17, Michal Prívozník wrote: > >>>> On 12/11/21 02:58, Qu Wenruo wrote: > >>>>> Hi, > >>>>> > >>>>> Recently I got my libvirt setup on both RK3399 (RockPro64) and RPI CM4, > >>>>> with upstream kernels. > >>>>> > >>>>> For RPI CM4 its mostly smooth sail, but on RK3399 due to its little.BIG > >>>>> setup (core 0-3 are 4x A55 cores, and core 4-5 are 2x A72 cores), it > >>>>> brings quite some troubles for VMs. > >>>>> > >>>>> In short, without proper cpuset to bind the VM to either all A72 cores > >>>>> or all A55 cores, the VM will mostly fail to boot. > > > > s/A55/A53/. There were thankfully no A72+A55 ever produced (just the > > though of it makes me sick). > > > >>>>> > >>>>> Currently the working xml is: > >>>>> > >>>>> <vcpu placement='static' cpuset='4-5'>2</vcpu> > >>>>> <cpu mode='host-passthrough' check='none'/> > >>>>> > >>>>> But even with vcpupin, pinning each vcpu to each physical core, VM will > >>>>> mostly fail to start up due to vcpu initialization failed with -EINVAL. > > > > Disclaimer: I know nothing about libvirt (and no, I don't want to > > know! ;-). > > > > However, for things to be reliable, you need to taskset the whole QEMU > > process to the CPU type you intend to use. > > Yep, that's what I'm doing. Are you sure? The xml directive above seem to only apply to the vcpus, and no other QEMU thread. > > That's because, AFAICT, > > QEMU will snapshot the system registers outside of the vcpu threads, > > and attempt to use the result to configure the actual vcpu threads. If > > they happen to run on different CPU types, the sysregs will differ in > > incompatible ways and an error will be returned. This may or may not > > be a bug, I don't know (I see it as a feature). > > Then this brings another question. > > If we can pin each vCPU to each physical core (both little and big), > then as long as the registers are per-vCPU based, it should be able to > pass both big and little cores to the VM. Absolutely. But that's not how QEMU works. It assumes that it can restore the *same* registers to all the vcpus. Which of course doesn't work (we don't allow you to change MIDR_EL1, for a start). > Yeah, I totally understand this screw up the scheduling, but that's at > least what (some insane) users want (just like me). That's fine, we all have our own use cases. > > > > > If you are annoyed with this behaviour, you can always use a different > > VMM that won't care about such difference (crosvm or kvmtool, to name > > a few). > > Sounds pretty interesting, a new world but without libvirt... > > > However, the guest will be able to observe the migration from > > one cpu type to another. This may or may not affect your guest's > > behaviour. > > Not sure if it's possible to pin each vCPU thread to each core, but let > me try. Again: the problem isn't the vcpu threads, but the dummy VM that QEMU creates to snapshot the vcpu registers. > > I personally find the QEMU behaviour reasonable. KVM/arm64 make little > > effort to support BL virtualisation as design choice (I value my > > sanity), and userspace is still in control of the placement. > > > >>>>> This brings a problem, in theory RK3399 SoC should out-perform BCM2711 > >>>>> in multi-core performance, but if a VM can only be bind to either A72 or > >>>>> A55 cores, then the performance is no longer competitive against > >>>>> BCM2711, wasting the PCIE 2.0 x4 capacity. > > > > Vote with your money. If you too think that BL systems are utter crap, > > do not buy them! Or treat them as 'two systems in one', which is what > > I do. From that angle, this is of great value! ;-) > > I guess I'm setting my expectation too high for rk3399, just seeing its > multi-thread perf beating RPI4 and has better IO doesn't mean it's a > perfect fit for VM. I find my own rk3399 perfectly adequate with QEMU. HTH, M. -- Without deviation from the norm, progress is not possible.
