Linux Virtualization - Aug 2020 - [PATCH v2 19/24] vdpa: make sure set_features in invoked for legacy

On 2020/8/4 ??5:00, Michael S. Tsirkin wrote:
> Some legacy guests just assume features are 0 after reset.
> We detect that config space is accessed before features are
> set and set features to 0 automatically.
> Note: some legacy guests might not even access config space, if this is
> reported in the field we might need to catch a kick to handle these.

I wonder whether it's easier to just support modern device?

Thanks

>
> Signed-off-by: Michael S. Tsirkin <mst at redhat.com>
> ---
>   drivers/vdpa/vdpa.c  |  1 +
>   include/linux/vdpa.h | 34 ++++++++++++++++++++++++++++++++++
>   2 files changed, 35 insertions(+)
>
> diff --git a/drivers/vdpa/vdpa.c b/drivers/vdpa/vdpa.c
> index de211ef3738c..7105265e4793 100644
> --- a/drivers/vdpa/vdpa.c
> +++ b/drivers/vdpa/vdpa.c
> @@ -96,6 +96,7 @@ struct vdpa_device *__vdpa_alloc_device(struct device
*parent,
>   	vdev->dev.release = vdpa_release_dev;
>   	vdev->index = err;
>   	vdev->config = config;
> +	vdev->features_valid = false;
>   
>   	err = dev_set_name(&vdev->dev, "vdpa%u",
vdev->index);
>   	if (err)
> diff --git a/include/linux/vdpa.h b/include/linux/vdpa.h
> index 239db794357c..29b8296f1414 100644
> --- a/include/linux/vdpa.h
> +++ b/include/linux/vdpa.h
> @@ -33,12 +33,14 @@ struct vdpa_notification_area {
>    * @dma_dev: the actual device that is performing DMA
>    * @config: the configuration ops for this device.
>    * @index: device index
> + * @features_valid: were features initialized? for legacy guests
>    */
>   struct vdpa_device {
>   	struct device dev;
>   	struct device *dma_dev;
>   	const struct vdpa_config_ops *config;
>   	unsigned int index;
> +	bool features_valid;
>   };
>   
>   /**
> @@ -266,4 +268,36 @@ static inline struct device *vdpa_get_dma_dev(struct
vdpa_device *vdev)
>   {
>   	return vdev->dma_dev;
>   }
> +
> +static inline void vdpa_reset(struct vdpa_device *vdev)
> +{
> +        const struct vdpa_config_ops *ops = vdev->config;
> +
> +	vdev->features_valid = false;
> +        ops->set_status(vdev, 0);
> +}
> +
> +static inline int vdpa_set_features(struct vdpa_device *vdev, u64
features)
> +{
> +        const struct vdpa_config_ops *ops = vdev->config;
> +
> +	vdev->features_valid = true;
> +        return ops->set_features(vdev, features);
> +}
> +
> +
> +static inline void vdpa_get_config(struct vdpa_device *vdev, unsigned
offset,
> +				   void *buf, unsigned int len)
> +{
> +        const struct vdpa_config_ops *ops = vdev->config;
> +
> +	/*
> +	 * Config accesses aren't supposed to trigger before features are
set.
> +	 * If it does happen we assume a legacy guest.
> +	 */
> +	if (!vdev->features_valid)
> +		vdpa_set_features(vdev, 0);
> +	ops->get_config(vdev, offset, buf, len);
> +}
> +
>   #endif /* _LINUX_VDPA_H */

On Wed, Aug 05, 2020 at 02:14:07PM +0800, Jason Wang
wrote:
> 
> On 2020/8/4 ??5:00, Michael S. Tsirkin wrote:
> > Some legacy guests just assume features are 0 after reset.
> > We detect that config space is accessed before features are
> > set and set features to 0 automatically.
> > Note: some legacy guests might not even access config space, if this
is
> > reported in the field we might need to catch a kick to handle these.
> 
> 
> I wonder whether it's easier to just support modern device?
> 
> Thanks

Well hardware vendors are I think interested in supporting legacy
guests. Limiting vdpa to modern only would make it uncompetitive.


> 
> > 
> > Signed-off-by: Michael S. Tsirkin <mst at redhat.com>
> > ---
> >   drivers/vdpa/vdpa.c  |  1 +
> >   include/linux/vdpa.h | 34 ++++++++++++++++++++++++++++++++++
> >   2 files changed, 35 insertions(+)
> > 
> > diff --git a/drivers/vdpa/vdpa.c b/drivers/vdpa/vdpa.c
> > index de211ef3738c..7105265e4793 100644
> > --- a/drivers/vdpa/vdpa.c
> > +++ b/drivers/vdpa/vdpa.c
> > @@ -96,6 +96,7 @@ struct vdpa_device *__vdpa_alloc_device(struct
device *parent,
> >   	vdev->dev.release = vdpa_release_dev;
> >   	vdev->index = err;
> >   	vdev->config = config;
> > +	vdev->features_valid = false;
> >   	err = dev_set_name(&vdev->dev, "vdpa%u",
vdev->index);
> >   	if (err)
> > diff --git a/include/linux/vdpa.h b/include/linux/vdpa.h
> > index 239db794357c..29b8296f1414 100644
> > --- a/include/linux/vdpa.h
> > +++ b/include/linux/vdpa.h
> > @@ -33,12 +33,14 @@ struct vdpa_notification_area {
> >    * @dma_dev: the actual device that is performing DMA
> >    * @config: the configuration ops for this device.
> >    * @index: device index
> > + * @features_valid: were features initialized? for legacy guests
> >    */
> >   struct vdpa_device {
> >   	struct device dev;
> >   	struct device *dma_dev;
> >   	const struct vdpa_config_ops *config;
> >   	unsigned int index;
> > +	bool features_valid;
> >   };
> >   /**
> > @@ -266,4 +268,36 @@ static inline struct device
*vdpa_get_dma_dev(struct vdpa_device *vdev)
> >   {
> >   	return vdev->dma_dev;
> >   }
> > +
> > +static inline void vdpa_reset(struct vdpa_device *vdev)
> > +{
> > +        const struct vdpa_config_ops *ops = vdev->config;
> > +
> > +	vdev->features_valid = false;
> > +        ops->set_status(vdev, 0);
> > +}
> > +
> > +static inline int vdpa_set_features(struct vdpa_device *vdev, u64
features)
> > +{
> > +        const struct vdpa_config_ops *ops = vdev->config;
> > +
> > +	vdev->features_valid = true;
> > +        return ops->set_features(vdev, features);
> > +}
> > +
> > +
> > +static inline void vdpa_get_config(struct vdpa_device *vdev, unsigned
offset,
> > +				   void *buf, unsigned int len)
> > +{
> > +        const struct vdpa_config_ops *ops = vdev->config;
> > +
> > +	/*
> > +	 * Config accesses aren't supposed to trigger before features
are set.
> > +	 * If it does happen we assume a legacy guest.
> > +	 */
> > +	if (!vdev->features_valid)
> > +		vdpa_set_features(vdev, 0);
> > +	ops->get_config(vdev, offset, buf, len);
> > +}
> > +
> >   #endif /* _LINUX_VDPA_H */

On 2020/8/5 ??7:40, Michael S. Tsirkin wrote:
> On Wed, Aug 05, 2020 at 02:14:07PM +0800, Jason Wang wrote:
>> On 2020/8/4 ??5:00, Michael S. Tsirkin wrote:
>>> Some legacy guests just assume features are 0 after reset.
>>> We detect that config space is accessed before features are
>>> set and set features to 0 automatically.
>>> Note: some legacy guests might not even access config space, if
this is
>>> reported in the field we might need to catch a kick to handle
these.
>> I wonder whether it's easier to just support modern device?
>>
>> Thanks
> Well hardware vendors are I think interested in supporting legacy
> guests. Limiting vdpa to modern only would make it uncompetitive.

My understanding is that, IOMMU_PLATFORM is mandatory for hardware vDPA 
to work. So it can only work for modern device ...

Thanks

>
>
>