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Date: Tue, 26 Nov 2013 18:08:27 +0200
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Subject: Re: [Qemu-devel] [RFC] create a single workqueue for each vm to
 update vm irq routing table
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To: Paolo Bonzini <pbonzini@redhat.com>
Cc: Avi Kivity <avi@cloudius-systems.com>, "Huangweidong (C)" <weidong.huang@huawei.com>, KVM <kvm@vger.kernel.org>, Gleb Natapov <gleb@redhat.com>, "Zhanghaoyu (A)" <haoyu.zhang@huawei.com>, Luonengjun <luonengjun@huawei.com>, "qemu-devel@nongnu.org" <qemu-devel@nongnu.org>, Zanghongyong <zanghongyong@huawei.com>, Avi Kivity <avi.kivity@gmail.com>, "Jinxin (F)" <jinxin712@huawei.com>

On Tue, Nov 26, 2013 at 04:58:53PM +0100, Paolo Bonzini wrote:
> Il 26/11/2013 16:35, Avi Kivity ha scritto:
> >>>> If we want to ensure, we need to use a different mechanism for
> >>>> synchronization than the global RCU.  QRCU would work; readers are not
> >>>> wait-free but only if there is a concurrent synchronize_qrcu, which
> >>>> should be rare.
> >>> An alternative path is to convince ourselves that the hardware does not
> >>> provide the guarantees that the current code provides, and so we can
> >>> relax them.
> >> No, I think it's a reasonable guarantee to provide.
> > 
> > Why?
> 
> Because IIUC the semantics may depend not just on the interrupt
> controller, but also on the specific PCI device.  It seems safer to
> assume that at least one device/driver pair wants this to work.
> 
> (BTW, PCI memory writes are posted, but configuration writes are not).
> 
> Paolo

You can also do a PCI read and flush out the writes.