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

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

>
>
>

On Thu, Aug 06, 2020 at 11:23:05AM +0800, Jason Wang
wrote:
> 
> 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.
Hmm I don't really see why. Assume host maps guest memory properly,
VM does not have an IOMMU, legacy guest can just work.

Care explaining what's wrong with this picture?

> So it can only work for modern device ...
> 
> Thanks
> 
> 
> > 
> > 
> >

On 2020/8/6 ??1:53, Michael S. Tsirkin wrote:
> On Thu, Aug 06, 2020 at 11:23:05AM +0800, Jason Wang wrote:
>> 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.
> Hmm I don't really see why. Assume host maps guest memory properly,
> VM does not have an IOMMU, legacy guest can just work.

Yes, guest may not set IOMMU_PLATFORM.

>
> Care explaining what's wrong with this picture?

The problem is virtio_vdpa, without IOMMU_PLATFORM it uses PA which can 
not work if IOMMU is enabled.

Thanks

>
>
>> So it can only work for modern device ...
>>
>> Thanks
>>
>>
>>>
>>>