On Tue, 14 Dec 2021 10:08:29 +0000, Peter Maydell <peter.maydell@linaro.org> wrote:
On Tue, 14 Dec 2021 at 09:52, Marc Zyngier <maz@kernel.org> wrote:
The best workaround is to taskset the QEMU process (and I really mean the process, not individual threads) to an homogeneous set of CPUs and be done with it.
Agreed. I suspect that often the 'little' CPUs are sufficiently low-power to probably not be worth giving to the VM anyway.
They are actually pretty beefy, in most cases. If you are I/O bound, the small cores are the right target. This email is brought to you via a couple of A55 CPUs running a VM and acting as my home gateway. If 'the internet' was getting slow, I'd have heard about it! ;-)
Side note: if you *do* give a guest both big and little CPUs using kvmtool or something similar, does the guest kernel get enough information to schedule tasks properly to both kinds of CPU, or does it just assume they're all the same and happily put performance-requiring tasks on the little CPUs ?
Upstream kvmtool doesn't make any effort to do this, but I had a series to perform such placement in the past. Once you know which physical CPUs you are placing the vcpus on, you can extract the individual CPU capacity[1] and expose it in the guest's DT as a cpu-specific 'capacity-dmips-mhz' property. A Linux guest will take the hint and do the right thing. M. [1] /sys/devices/system/cpu/cpu*/cpu_capacity -- Without deviation from the norm, progress is not possible.