Re: [virtio-dev] Re: [RFC] content: tweak VIRTIO_F_IO_BARRIER

[Jason Wang <jasowang@redhat.com>]

On 2018年06月29日 12:21, Michael S. Tsirkin wrote:
> > > The discussion is to talk about how to fix the potential
> > > problems that currently may happen when the virtio devices
> > > (which need to bypass the IOMMU) work on the platforms
> > > (which have DMA limitations).
> > >
> > > Currently,
> > >
> > > - IO_BARRIER is just to tell drivers which type of barriers
> > >     should be used.
> > >
> > > - IOMMU_PLATFORM (from my understanding) is to tell drivers
> > >     whether the devices need to bypass the IOMMU.
> > >
> > > Michael is asking whether we should tweak above two bits
> > > or whether we should do something else to solve this problem.
> > >
> > > If we want to tweak above two bits, they may become
> > > something like:
> > >
> > > - PLATFORM_CACHE (from IO_BARRIER): about the memory
> > >      operations visibility between driver and device.
> > One issue here is sometime we may have something like a software IOTLB which
> > is transparent to device. Just infer from the name of PLATFORM_CACHE, it
> > seems does not cover this case.
> I don't think device needs to know about that.
> That's up to driver/OS.

So the question is can driver get those limitations or requirements 
through a transport specific way instead of depending a feature. If we 
define a feature that does not cover all the cases, we probably need 
another one to fix it. That's suboptimal.

> > > - PLATFORM_IOMMU (from IOMMU_PLATFORM): about whether
> > >      the DMA addr passed to the device should be prepared
> > >      (because e.g. the device is behind an IOMMU, or the
> > >      device can only access parts of system memory).
> > So I think it's a bug of driver instead of a missing feature.
> what is?

I mean it was probably a bug that we must specify PLATFORM_IOMMU to use 
swiotlb.

> > We can even
> > fail the device probe in this case for safety.
> >
> > Thanks
> device can and does.

Yes.

Thanks

> > > Currently, I don't know what's the best choice..
> > >
> > > Best regards,
> > > Tiwei Bie