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

[Robin Murphy]

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?

> > 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 :)

Robin.

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