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Date: Tue, 26 Nov 2013 16:58:53 +0100
From: Paolo Bonzini <pbonzini@redhat.com>
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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: Avi Kivity <avi@cloudius-systems.com>
Cc: "Huangweidong (C)" <weidong.huang@huawei.com>, KVM <kvm@vger.kernel.org>, Gleb Natapov <gleb@redhat.com>, "Michael S. Tsirkin" <mst@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>

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