On Wed, 22 Nov 2023, Mihai Carabas wrote:
La 22.11.2023 22:51, Christoph Lameter a scris:
On Mon, 20 Nov 2023, Mihai Carabas wrote:
cpu_relax on ARM64 does a simple "yield". Thus we replace it with
smp_cond_load_relaxed which basically does a "wfe".
Well it clears events first (which requires the first WFE) and then does a
WFE waiting for any events if no events were pending.
WFE does not cause a VMEXIT? Or does the inner loop of
smp_cond_load_relaxed now do 2x VMEXITS?
KVM ARM64 code seems to indicate that WFE causes a VMEXIT. See
kvm_handle_wfx().
In KVM ARM64 the WFE traping is dynamic: it is enabled only if there are more
tasks waiting on the same core (e.g. on an oversubscribed system).
In arch/arm64/kvm/arm.c:
457 >-------if (single_task_running())
458 >------->-------vcpu_clear_wfx_traps(vcpu);
459 >-------else
460 >------->-------vcpu_set_wfx_traps(vcpu);
Ahh. Cool did not know about that. But still: Lots of VMEXITs once the
load has to be shared.
This of course can be improved by having a knob where you can completly
disable wfx traping by your needs, but I left this as another subject to
tackle.
kvm_arch_vcpu_load() looks strange. On the one hand we pass a cpu
number into it and then we use functions that only work if we are running
on that cpu?
It would be better to use smp_processor_id() in the function
and not pass the cpu number to it.
