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From: Michael Tokarev <mjt@tls.msk.ru>
To: Anthony Liguori <anthony@codemonkey.ws>
Cc: Saul Tamari <stamari@gmail.com>, kvm <kvm@vger.kernel.org>
Subject: Re: block virtio excessive VIRTIO_PCI_ISR reads
Date: Wed, 28 Oct 2009 17:59:33 +0300	[thread overview]
Message-ID: <4AE85C55.7010806@msgid.tls.msk.ru> (raw)
In-Reply-To: <4AE859BD.106@codemonkey.ws>

Anthony Liguori wrote:
> Saul Tamari wrote:
>> Hi,
>>
>> There seems to be a bug in the VirtIO guest device driver (tested with
>> 2.6.31.5 kernel).
>> The bug causes frequent & excessive VIRTIO_PCI_ISR reads (causing PIO
>> vm-exits) for a device that is not being used.
>>
>> The setup I’m using includes two VirtIO block devices – vda & vdb.
>> While running heavy IO on vdb I notice a lot of VIRTIO_PCI_ISR port
>> reads for the PIO ports of vda too.
>> For each vdb IO operation passed to the hypervisor by the virtio 
>> driver I see:
>> 1. One VIRTIO_PCI_QUEUE_NOTIFY write on vdb
>> 2. One VIRTIO_PCI_ISR read on vdb
>> 3. One VIRTIO_PCI_ISR read on vda (while not explicitly using vda)
> 
> That's because it's using a shared interrupt and each device needs to 
> check whether it's got pending work to do.
> 
> You can eliminate this by using MSI-x.

So the next question is how to use MSI-x.. ;)

Thanks!

/mjt

  reply	other threads:[~2009-10-28 14:59 UTC|newest]

Thread overview: 9+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2009-10-28 14:40 block virtio excessive VIRTIO_PCI_ISR reads Saul Tamari
2009-10-28 14:48 ` Anthony Liguori
2009-10-28 14:59   ` Michael Tokarev [this message]
2009-10-28 15:04     ` Saul Tamari
2009-10-28 15:52       ` Saul Tamari
2009-10-29 10:10       ` Avi Kivity
2009-10-29 15:47         ` Saul Tamari
2009-11-01  9:23           ` Avi Kivity
2009-11-01 11:16             ` Saul Tamari

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