RE: [Xen-devel] evtchn_upcall_mask for PV-on-HVM

To:	"Keir Fraser" <keir@xxxxxxxxxxxxx>, <xen-devel@xxxxxxxxxxxxxxxxxxx>
Subject:	RE: [Xen-devel] evtchn_upcall_mask for PV-on-HVM
From:	"Tian, Kevin" <kevin.tian@xxxxxxxxx>
Date:	Fri, 1 Dec 2006 10:29:24 +0800
Cc:	"Jiang, Yunhong" <yunhong.jiang@xxxxxxxxx>
Delivery-date:	Thu, 30 Nov 2006 18:29:30 -0800
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Thread-topic:	[Xen-devel] evtchn_upcall_mask for PV-on-HVM

>From: Keir Fraser [mailto:keir@xxxxxxxxxxxxx]
>Sent: 2006年11月30日 22:51
>> what's the block() wakeup-waiting race above? :-)
>
>I mean that the evtchn_upcall_pending flag is not the master flag
>indicating
>whether there are pending interrupts for an HVM guest. So it is not the
>correct thing to test in local_events_need_delivery(). The correct thing to
>test is whether there are any interrupts pending from the PIC/IOAPIC.
>
>The wakeup-waiting race is that a VCPU may block just when, for
>example, the
>qemu-dm rtl8139 device model is asserting an interrupt. It will try to
>vcpu_kick() and send an IPI to the CPU that is running the HVM VCPU.
>However, before that IPI is received the other CPU may block the HVM
>VCPU
>(after checking for local_events_need_delivery() which will in this case
>return 0). So the rtl8139 interrupt does not prevent the HVM VCPU from
>blocking, which is a bug.
>
> - Keir

OK, I see. That's a real performance bug..

Thanks for explanation. :-)

Thanks,
Kevin

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