Re: [Qemu-devel] [PATCH v4] virtio-pci: error out when both legacy and modern modes are disabled

[Greg Kurz]

On Fri, 22 Jul 2016 15:42:48 +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.
> > >   
> > 
> > Isn't it already what happens with legacy being the default in pre-2.7 QEMU 
> > ?  
> 
> Well, that is exactly my point; users may be surprised.
> 

One day legacy will be hopefully buried :)

> > 
> > Do you think we should have separate checks for pcie and !pcie ?  
> 
> I don't think we should overengineer this.
> 

Agreed.

> >   
> > > >     
> > > > > > +        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).
> > >   
> > 
> > Thanks ! This is THE wording I was looking for :)  
> 
> :)
> 
> I'm fine with the patch with the changed wording, as it less confusing
> for the user.
> 
> <I'll be offline next week so please just attach my
> 
> Reviewed-by: Cornelia Huck <address@hidden>
> 
> if the only thing you change is the message>
> 

I'll do this right away as I'll be offline for 1 month starting... just after
I post v5 :)

Cheers.

--
Greg