From mboxrd@z Thu Jan  1 00:00:00 1970
From: Anthony Liguori <anthony@codemonkey.ws>
Subject: Re: block virtio excessive VIRTIO_PCI_ISR reads
Date: Wed, 28 Oct 2009 09:48:29 -0500
Message-ID: <4AE859BD.106@codemonkey.ws>
References: <41be791c0910280740t7942bb7cj692ba54725bcc5eb@mail.gmail.com>
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Cc: kvm <kvm@vger.kernel.org>
To: Saul Tamari <stamari@gmail.com>
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Saul Tamari wrote:
> Hi,
>
> There seems to be a bug in the VirtIO guest device driver (tested wit=
h
> 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=92m using includes two VirtIO block devices =96 vda & vdb=
=2E
> 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 driv=
er 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)
>  =20

That's because it's using a shared interrupt and each device needs to=20
check whether it's got pending work to do.

You can eliminate this by using MSI-x.

Regards,

Anthony Liguori