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Re: [Qemu-devel] [PATCH v4] virtio-pci: error out when both legacy and m


From: Cornelia Huck
Subject: Re: [Qemu-devel] [PATCH v4] virtio-pci: error out when both legacy and modern modes are disabled
Date: Fri, 22 Jul 2016 12:32:24 +0200

On Fri, 22 Jul 2016 12:11:11 +0200
Greg Kurz <address@hidden> wrote:

> On Fri, 22 Jul 2016 10:04:35 +0200
> Cornelia Huck <address@hidden> wrote:
> 
> > On Thu, 21 Jul 2016 23:21:16 +0200
> > Greg Kurz <address@hidden> wrote:
> > 
> > > From: Greg Kurz <address@hidden>
> > > 
> > > Without presuming if we got there because of a user mistake or some
> > > more subtle bug in the tooling, it really does not make sense to
> > > implement a non-functional device.
> > > 
> > > Signed-off-by: Greg Kurz <address@hidden>
> > > Reviewed-by: Marcel Apfelbaum <address@hidden>
> > > Signed-off-by: Greg Kurz <address@hidden>
> > > ---
> > > v4: - rephrased error message and provide a hint to the user
> > >     - split string literals to stay below 80 characters
> > >     - added Marcel's R-b tag
> > > ---
> > >  hw/virtio/virtio-pci.c |    8 ++++++++
> > >  1 file changed, 8 insertions(+)
> > > 
> > > diff --git a/hw/virtio/virtio-pci.c b/hw/virtio/virtio-pci.c
> > > index 755f9218b77d..72c4b392ffda 100644
> > > --- a/hw/virtio/virtio-pci.c
> > > +++ b/hw/virtio/virtio-pci.c
> > > @@ -1842,6 +1842,14 @@ static void virtio_pci_dc_realize(DeviceState 
> > > *qdev, Error **errp)
> > >      VirtIOPCIProxy *proxy = VIRTIO_PCI(qdev);
> > >      PCIDevice *pci_dev = &proxy->pci_dev;
> > > 
> > > +    if (!(virtio_pci_modern(proxy) || virtio_pci_legacy(proxy))) {  
> > 
> > I'm not sure that I didn't mess up the sequence of the realize
> > callbacks, but could disable_legacy still be AUTO here? In that case,
> > we'd fail for disable-modern=on and disable-legacy unset (i.e., AUTO),
> > which would be ok for pcie but not for !pcie.
> > 
> 
> Marcel made the same comment in:
> 
> https://lists.gnu.org/archive/html/qemu-devel/2016-07/msg05225.html
> 
> If the user explicitly disables modern, she shouldn't rely on QEMU
> implicitly enabling legacy, hence the suggestion in error_append_hint().

I don't know, I'd find that a bit surprising, especially as I would end
up with a legacy-capable device if I did not specify anything in
the !pcie case.

> 
> > > +        error_setg(errp, "device cannot work when both modern and legacy 
> > > modes"
> > > +                   " are disabled");

Suggest to change this wording to:

"device cannot work as neither modern nor legacy mode is enabled"

as this more accurately reflects what happened (the user did not
actively disable legacy in the case above).

> > > +        error_append_hint(errp, "Set either disable-modern or 
> > > disable-legacy"
> > > +                          " to off\n");

The hint looks fine to me :)

> > > +        return;
> > > +    }
> > > +
> > >      if (!(proxy->flags & VIRTIO_PCI_FLAG_DISABLE_PCIE) &&
> > >          virtio_pci_modern(proxy)) {
> > >          pci_dev->cap_present |= QEMU_PCI_CAP_EXPRESS;
> > >   
> > 
> 




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