Re: [PATCH 4/5] iommu/vt-d: Disallow SVA if devices don't support 64-bit address

[Lu Baolu]

On 2021/7/21 19:12, Robin Murphy wrote:
> On 2021-07-21 02:50, Lu Baolu wrote:
> > Hi Robin,
> >
> > Thanks a lot for reviewing my patch!
> >
> > On 7/20/21 5:27 PM, Robin Murphy wrote:
> > > On 2021-07-20 02:38, Lu Baolu wrote:
> > > > When the device and CPU share an address space (such as SVA), the 
> > > > device
> > > > must support the same addressing capability as the CPU. The CPU does 
> > > > not
> > > > consider the addressing ability of any device when managing the page 
> > > > table
> > > > of a process, so the device must have enough addressing ability to bind
> > > > the page table of the process.
> > > >
> > > > Signed-off-by: Lu Baolu <baolu.lu@xxxxxxxxxxxxxxx>
> > > > ---
> > > >   drivers/iommu/intel/iommu.c | 3 +++
> > > >   1 file changed, 3 insertions(+)
> > > >
> > > > diff --git a/drivers/iommu/intel/iommu.c b/drivers/iommu/intel/iommu.c
> > > > index f45c80ce2381..f3cca1dd384d 100644
> > > > --- a/drivers/iommu/intel/iommu.c
> > > > +++ b/drivers/iommu/intel/iommu.c
> > > > @@ -5372,6 +5372,9 @@ static int intel_iommu_enable_sva(struct 
> > > > device *dev)
> > > >       if (!(iommu->flags & VTD_FLAG_SVM_CAPABLE))
> > > >           return -ENODEV;
> > > > +    if (!dev->dma_mask || *dev->dma_mask != DMA_BIT_MASK(64))
> > >
> > > Careful - VFIO doesn't set DMA masks (since it doesn't use the DMA API),
> >
> > SVA doesn't work through the VFIO framework.
>
> Did anyone say it does? My point is that, as far as I understand, the 
> SVA UAPI is very much intended to work *with* VFIO, and even if the 
> finer details are still mired in the /dev/ioasid discussion today we 
> should definitely expect to see VFIO-like use-cases at some point. I 
> certainly don't see why any of the guest SVA stuff exists already if not 
> for VFIO-assigned devices?

Agreed. From /dev/ioasid design point of view, it's possible to have
VFIO-like use case of SVA. Perhaps the device addressing capability
could be included in GET_DEV_INFO of /dev/ioasid UAPI.

> > > so this appears to be relying on another driver having bound previously,
> >
> > Yes. You are right.
> >
> > > otherwise the mask would still be the default 32-bit one from 
> > > pci_setup_device(). I'm not sure that's an entirely robust assumption.
> >
> > Currently SVA implementation always requires a native kernel driver. The
> > assumption is that the drivers should check and set 64-bit addressing
> > capability before calling iommu_sva_xxx() APIs.
>
> ...and given that that is not a documented requirement, and certainly 
> not a technical one (even a self-contained kernel driver could choose to 
> only use SVA contexts and not touch the DMA API), it's an inherently 
> fragile assumption which I'm confident *will* be broken eventually :)

Fair enough. I will drop this patch.

Thanks a lot for the comments.

Best regards,
baolu

> Robin.
>
> > > > +        return -ENODEV;
> > > > +
> > > >       if (intel_iommu_enable_pasid(iommu, dev))
> > > >           return -ENODEV;
> >
> > Best regards,
> > baolu