Re: [Qemu-devel] [PATCH v2 1/2] hw: report invalid disable-legacy|modern usage for virtio-1-only devs

["Daniel P. Berrangé" <berrange@redhat.com>]

On Fri, May 17, 2019 at 04:01:29PM -0300, Eduardo Habkost wrote:
> Hi,
> 
> Sorry for taking so long to look at this more closely:
> 
> On Fri, Feb 15, 2019 at 10:32:38AM +0000, Daniel P. Berrangé wrote:
> > A number of virtio devices (gpu, crypto, mouse, keyboard, tablet) only
> > support the virtio-1 (aka modern) mode. Currently if the user launches
> > QEMU, setting those devices to enable legacy mode, QEMU will silently
> > create them in modern mode, ignoring the user's (mistaken) request.
> > 
> > This patch introduces proper data validation so that an attempt to
> > configure a virtio-1-only devices in legacy mode gets reported as an
> > error to the user.
> > 
> > Checking this required introduction of a new field to explicitly track
> > what operating model is to be used for a device, separately from the
> > disable_modern and disable_legacy fields that record the user's
> > requested configuration.
> 
> I'm still trying to understand why we need to add a new field.
> 
> If disable_modern, disable_legacy and mode are always expected to
> be consistent with each other, why do we need another field?
> 
> If they are not always consistent with each other, when exactly
> do we want them to be inconsistent, and why?

The pain point is that we're using the existing variables to record
two distinct pieces of information

 - The user's request for modern vs legacy
 - The PCI bus requirements for modern vs legacy

The existing code would overwrite the user's setting for
"disable_legacy" when deciding whether the device is in
a PCI or PCIe port. This happens in virtio_pci_realize.

We can only report errors with the user's requested config
after the sub-classes call virtio_pci_force_virtio_1, but
this doesn't happen until virtio_${subclass}_pci_release.

So by the time we're able to report errors, virtio_pci_realize
has already overwritten the user's disable_legacy setting, so
we've lost the very piece of info we need to check to report
errors with.

Given the ordering of virtio_pci_realize vs the calls
to virtio_pci_force_virtio_1 by subclasses, I don't see any
option other than to use separate variables for the two
distinct pieces of information.

> 
> > 
> > Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
> [...]
> > diff --git a/hw/virtio/virtio-pci.h b/hw/virtio/virtio-pci.h
> > index bd223a6e3b..16ef4c0a3f 100644
> > --- a/hw/virtio/virtio-pci.h
> > +++ b/hw/virtio/virtio-pci.h
> > @@ -15,6 +15,7 @@
> >  #ifndef QEMU_VIRTIO_PCI_H
> >  #define QEMU_VIRTIO_PCI_H
> >  
> > +#include "qapi/error.h"
> >  #include "hw/pci/msi.h"
> >  #include "hw/virtio/virtio-bus.h"
> >  
> > @@ -118,6 +119,12 @@ typedef struct VirtIOPCIQueue {
> >    uint32_t used[2];
> >  } VirtIOPCIQueue;
> >  
> > +typedef enum {
> > +    VIRTIO_PCI_MODE_LEGACY,
> > +    VIRTIO_PCI_MODE_TRANSITIONAL,
> > +    VIRTIO_PCI_MODE_MODERN,
> > +} VirtIOPCIMode;
> > +
> >  struct VirtIOPCIProxy {
> >      PCIDevice pci_dev;
> >      MemoryRegion bar;
> > @@ -142,6 +149,7 @@ struct VirtIOPCIProxy {
> >      bool disable_modern;
> >      bool ignore_backend_features;
> >      OnOffAuto disable_legacy;
> > +    VirtIOPCIMode mode;
> >      uint32_t class_code;
> >      uint32_t nvectors;
> >      uint32_t dfselect;
> > @@ -156,23 +164,34 @@ struct VirtIOPCIProxy {
> >  
> >  static inline bool virtio_pci_modern(VirtIOPCIProxy *proxy)
> >  {
> > -    return !proxy->disable_modern;
> > +    return proxy->mode != VIRTIO_PCI_MODE_LEGACY;
> >  }
> >  
> >  static inline bool virtio_pci_legacy(VirtIOPCIProxy *proxy)
> >  {
> > -    return proxy->disable_legacy == ON_OFF_AUTO_OFF;
> > +    return proxy->mode != VIRTIO_PCI_MODE_MODERN;
> >  }
> >  
> > -static inline void virtio_pci_force_virtio_1(VirtIOPCIProxy *proxy)
> > +static inline bool virtio_pci_force_virtio_1(VirtIOPCIProxy *proxy,
> > +                                             Error **errp)
> >  {
> > -    proxy->disable_modern = false;
> > -    proxy->disable_legacy = ON_OFF_AUTO_ON;
> > +    if (proxy->disable_legacy == ON_OFF_AUTO_OFF) {
> > +        error_setg(errp, "Unable to set disable-legacy=off on a virtio-1.0 "
> > +                   "only device");
> > +        return false;
> > +    }
> > +    if (proxy->disable_modern == true) {
> > +        error_setg(errp, "Unable to set disable-modern=on on a virtio-1.0 "
> > +                   "only device");
> > +        return false;
> > +    }
> > +    proxy->mode = VIRTIO_PCI_MODE_MODERN;
> > +    return true;
> >  }
> >  
> >  static inline void virtio_pci_disable_modern(VirtIOPCIProxy *proxy)
> >  {
> > -    proxy->disable_modern = true;
> > +    proxy->mode = VIRTIO_PCI_MODE_LEGACY;
> >  }
> >  
> >  /*
> > -- 
> > 2.20.1
> > 
> > 
> 
> -- 
> Eduardo
> 

Regards,
Daniel
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