Re: [PATCH v3 0/4] implement vcpu preempted check

From: Pan Xinhui
Date: Fri Sep 30 2016 - 04:53:25 EST

hi, Paolo
thanks for your reply.

å 2016/9/30 14:58, Paolo Bonzini åé:
Please consider s390 and (x86/arm) KVM. Once we have a few, more can
follow later, but I think its important to not only have PPC support for

Actually the s390 preemted check via sigp sense running is available for
all hypervisors (z/VM, LPAR and KVM) which implies everywhere as you can
no longer buy s390 systems without LPAR.

As Heiko already pointed out we could simply use a small inline function
that calls cpu_is_preempted from arch/s390/lib/spinlock (or
smp_vcpu_scheduled from smp.c)

Sure, and I had vague memories of Heiko's email. This patch set however
completely fails to do that trivial hooking up.

sorry for that.
I will try to work it out on x86.

x86 has no hypervisor support, and I'd like to understand the desired
semantics first, so I don't think it should block this series. In

Once a guest do a hypercall or something similar, IOW, there is a kvm_guest_exit. we think this is a lock holder preemption.
Adn PPC implement it in this way.

particular, there are at least the following choices:

1) exit to userspace (5-10.000 clock cycles best case) counts as
lock holder preemption

just to avoid any misunderstanding.
You are saying that the guest does an IO operation for example and then exit to QEMU right?
Yes, in this scenario it's hard to guarntee that such IO operation or someghing like that could be finished in time.

2) any time the vCPU thread not running counts as lock holder

To implement the latter you'd need a hypercall or MSR (at least as
a slow path), because the KVM preempt notifier is only active
during the KVM_RUN ioctl.

seems a little expensive. :(
How many clock cycles it might cost.

I am still looking for one shared struct between kvm and guest kernel on x86.
and every time kvm_guest_exit/enter called, we store some info in it. So guest kernel can check one vcpu is running or not quickly.