On 07/11/2012 02:18 PM, Christian Borntraeger wrote:On 11/07/12 13:04, Avi Kivity wrote:On 07/11/2012 01:17 PM, Christian Borntraeger wrote:On 11/07/12 11:06, Avi Kivity wrote:
[...]
Almost all s390 kernels use diag9c (directed yield to a given guest cpu) for spinlocks, though.
Perhaps x86 should copy this.
See arch/s390/lib/spinlock.c
The basic idea is using several heuristics:
- loop for a given amount of loops
- check if the lock holder is currently scheduled by the hypervisor
(smp_vcpu_scheduled, which uses the sigp sense running instruction)
Dont know if such thing is available for x86. It must be a lot cheaper
than a guest exit to be useful
We could make it available via shared memory, updated using preempt
notifiers. Of course piling on more pv makes this less attractive.
- if lock holder is not running and we looped for a while do a directed
yield to that cpu.
So there is no win here, but there are other cases were diag44 is used, e.g. cpu_relax.
I have to double check with others, if these cases are critical, but for now, it seems
that your dummy implementation for s390 is just fine. After all it is a no-op until
we implement something.
Does the data structure make sense for you? If so we can move it to
common code (and manage it in kvm_vcpu_on_spin()). We can guard it with
CONFIG_KVM_HAVE_CPU_RELAX_INTERCEPT or something, so other archs don't
have to pay anything.
Ignoring the name,
What name would you suggest?
maybe vcpu_no_progress instead of pause_loop_exited
Ah, I thouht you objected to the CONFIG var. Maybe call it
cpu_relax_intercepted since that's the linuxy name for the instruction.