Re: [PATCH V2 3/5] ara virt interface of perf to support kvm guest os statistics collection in guest os

Zhang, Yanmin
2010-06-22T05:47:16Z

On Mon, 2010-06-21 at 16:56 +0300, Gleb Natapov wrote:
> On Mon, Jun 21, 2010 at 05:31:43PM +0800, Zhang, Yanmin wrote:
> > The 3rd patch is to implement para virt perf at host kernel.
> >
> > Signed-off-by: Zhang Yanmin <yanmin_zhang@linux.intel.com>
> >
<snip>

> > +
> > +static void kvm_copy_event_to_guest(struct kvm_vcpu *vcpu,
> > + struct perf_event *host_event)
> > +{
> > + struct host_perf_shadow *shadow = host_event->host_perf_shadow;
> > + struct guest_perf_event counter;
> > + int ret;
> > + s32 overflows;
> > +
> > + ret = kvm_read_guest(vcpu->kvm, shadow->guest_event_addr,
> > + &counter, sizeof(counter));
> > + if (ret < 0)
> > + return;
> > +
> > +again:
> > + overflows = atomic_read(&shadow->counter.overflows);
> > + if (atomic_cmpxchg(&shadow->counter.overflows, overflows, 0) !=
> > + overflows)
> > + goto again;
> > +
> > + counter.count = shadow->counter.count;
> > + atomic_add(overflows, &counter.overflows);
> > +
> > + kvm_write_guest(vcpu->kvm,
> > + shadow->guest_event_addr,
> > + &counter,
> > + sizeof(counter));
> Those kind of interfaces worry me since the can cause bugs that are
> very hard to catch. What if guest enables some events and crashes into
> kdump kernel (or kexec new kernel) without reseting HW. Now host may
> write over guest memory without guest expecting it. Do you handle this
> scenario in a guest side? I think you need to register reboot notify
> and disable events from there.
Sorry for missing your comments.

My patch could take care of dead guest os by cleaning up all events in function
kvm_arch_destroy_vm, so all events are closed if host user kills the guest
qemu process.

As for your scenario, I will register reboot notify and add a new pv perf
hypercall interface to vmexit to host kernel to do cleanup.

Thanks,
Yanmin