On 2014-07-04 08:17, Wanpeng Li wrote: > On Thu, Jul 03, 2014 at 01:15:26AM -0400, Bandan Das wrote: >> Jan Kiszka <jan.kiszka@xxxxxxxxxxx> writes: >> >>> On 2014-07-02 08:54, Wanpeng Li wrote: >>>> This patch fix bug https://bugzilla.kernel.org/show_bug.cgi?id=72381 >>>> >>>> If we didn't inject a still-pending event to L1 since nested_run_pending, >>>> KVM_REQ_EVENT should be requested after the vmexit in order to inject the >>>> event to L1. However, current log blindly request a KVM_REQ_EVENT even if >>>> there is no still-pending event to L1 which blocked by nested_run_pending. >>>> There is a race which lead to an interrupt will be injected to L2 which >>>> belong to L1 if L0 send an interrupt to L1 during this window. >>>> >>>> VCPU0 another thread >>>> >>>> L1 intr not blocked on L2 first entry >>>> vmx_vcpu_run req event >>>> kvm check request req event >>>> check_nested_events don't have any intr >>>> not nested exit >>>> intr occur (8254, lapic timer etc) >>>> inject_pending_event now have intr >>>> inject interrupt >>>> >>>> This patch fix this race by introduced a l1_events_blocked field in nested_vmx >>>> which indicates there is still-pending event which blocked by nested_run_pending, >>>> and smart request a KVM_REQ_EVENT if there is a still-pending event which blocked >>>> by nested_run_pending. >>> >>> There are more, unrelated reasons why KVM_REQ_EVENT could be set. Why >>> aren't those able to trigger this scenario? >>> >>> In any case, unconditionally setting KVM_REQ_EVENT seems strange and >>> should be changed. >> >> >> Ugh! I think I am hitting another one but this one's probably because >> we are not setting KVM_REQ_EVENT for something we should. >> >> Before this patch, I was able to hit this bug everytime with >> "modprobe kvm_intel ept=0 nested=1 enable_shadow_vmcs=0" and then booting >> L2. I can verify that I was indeed hitting the race in inject_pending_event. >> >> After this patch, I believe I am hitting another bug - this happens >> after I boot L2, as above, and then start a Linux kernel compilation >> and then wait and watch :) It's a pain to debug because this happens >> almost once in three times; it never happens if I run with ept=1, however, >> I think that's only because the test completes sooner. But I can confirm >> that I don't see it if I always set REQ_EVENT if nested_run_pending is set instead of >> the approach this patch takes. >> (Any debug hints help appreciated!) >> >> So, I am not sure if this is the right fix. Rather, I think the safer thing >> to do is to have the interrupt pending check for injection into L1 at >> the "same site" as the call to kvm_queue_interrupt() just like we had before >> commit b6b8a1451fc40412c57d1. Is there any advantage to having all the >> nested events checks together ? >> > > How about revert commit b6b8a1451 and try if the bug which you mentioned > is still there? I suspect you will have to reset back to b6b8a1451^ for this as other changes depend on this commit now. Jan -- Siemens AG, Corporate Technology, CT RTC ITP SES-DE Corporate Competence Center Embedded Linux -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html