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Date: Fri, 4 Dec 2015 11:15:48 +0100
From: Cornelia Huck <cornelia.huck@de.ibm.com>
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Subject: Re: [Qemu-devel] [2.5 issue] virtio-1 in virtio-net and old vhost
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To: Jason Wang <jasowang@redhat.com>
Cc: Christian Borntraeger <borntraeger@de.ibm.com>, qemu-devel@nongnu.org, "Michael S. Tsirkin" <mst@redhat.com>

On Fri, 4 Dec 2015 10:06:58 +0800
Jason Wang <jasowang@redhat.com> wrote:

> The problem is for pci: without this patch, guest may always see modern
> bar is "disable-modern=false". But with this patch, on an old kernel
> that does not support VERSION_1, even "disable-modern=false" were
> specified, guest can not see modern bar anymore. Looks like a guest
> visible change.

But the guest even see a modern bar if the host is not able to present
a spec-compliant device?

What we have now is a device that looks like a modern device but that
does not offer VERSION_1. Probably not spec compliant. If virtio-pci is
fixed to not present such broken devices to the guest, this is
guest-visible, yes; but only in the sense that the guest will no longer
see broken devices, but simply legacy devices if configured.