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Date:   Tue, 20 Aug 2019 09:22:09 -0600
From:   Alex Williamson <alex.williamson@redhat.com>
To:     luoben <luoben@linux.alibaba.com>
Cc:     tglx@linutronix.de, linux-kernel@vger.kernel.org,
        tao.ma@linux.alibaba.com, gerry@linux.alibaba.com,
        nanhai.zou@linux.alibaba.com, linyunsheng@huawei.com
Subject: Re: [PATCH v3 0/3] genirq/vfio: Introduce update_irq_devid and
 optimize VFIO irq ops
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On Tue, 20 Aug 2019 12:03:50 +0800
luoben <luoben@linux.alibaba.com> wrote:

> 在 2019/8/20 上午4:51, Alex Williamson 写道:
> > On Thu, 15 Aug 2019 21:02:58 +0800
> > Ben Luo <luoben@linux.alibaba.com> wrote:
> >  
> >> Currently, VFIO takes a lot of free-then-request-irq actions whenever
> >> a VM (with device passthru via VFIO) sets irq affinity or mask/unmask
> >> irq. Those actions only change the cookie data of irqaction or even
> >> change nothing. The free-then-request-irq not only adds more latency,
> >> but also increases the risk of losing interrupt, which may lead to a
> >> VM hung forever in waiting for IO completion  
> > What guest environment is generating this?  Typically I don't see that
> > Windows or Linux guests bounce the interrupt configuration much.
> > Thanks,
> >
> > Alex  
> 
> By tracing centos5u8 on host, I found it keep masking and unmasking 
> interrupt like this:
> 
> [1566032533709879] index:28 irte_hi:000000010004a601 
> irte_lo:adb54bc000b98001
> [1566032533711242] index:28 irte_hi:0000000000000000 
> irte_lo:0000000000000000
> [1566032533711258] index:28 irte_hi:000000000004a601 
> irte_lo:00003fff00ac002d
> [1566032533711269] index:28 irte_hi:000000000004a601 
> irte_lo:00003fff00ac002d
[snip] 
> "[1566032533720007]" is timestamp in μs, so centos5u8 tiggers 30+ irte 
> modification within 10ms

Ok, that matches my understanding that only very old guests behave in
this manner.  It's a curious case to optimize as RHEL5 is in extended
life-cycle support, with regular maintenance releases ending 2+ years
ago.  Thanks,

Alex