Re: [RFC PATCH v2 00/58] KVM: Arm SMMUv3 driver for pKVM

[Mostafa Saleh <smostafa@google.com>]

On Wed, Jan 22, 2025 at 12:20:55PM -0400, Jason Gunthorpe wrote:
> On Wed, Jan 22, 2025 at 11:04:24AM +0000, Mostafa Saleh wrote:
> > AFAIK, the most common use cases would be:
> > - Devices using DMA API because it requires a lot of memory to be
> >   contiguous in IOVA, which is hard to do with identity
> 
> This is not a feature of the DMA API any driver should rely on .. Are
> you aware of one that does?
> 

I’d guess one example is media drivers, they usually need large contiguous
buffers, and would use for ex dma_alloc_coherent(), if the IOMMU is disabled or
bypassed, that means that the kernel has to find such contiguous size in the
physical address which can be impossible on devices with small memory as
mobile devices. Similarly.

I will look more into this while working on the patches to identity map
everything for v3, and I’d see what kind of issues I hit.

> > - Devices with security requirements/constraints to be isolated from the
> >   rest of the system, also using DMA API
> 
> This is real, but again, in a mobile context does this even exist? It isn't
> like there are external PCIe ports that need securing on a phone?

It’s not just about completely external devices, it’s a defence in depth
measure, where for example, network devices can be poked externally an
there have cases in the past where exploits were found[1], so some vendors
might have a policy to isolate such devices. Which I believe is a valid.

[1] https://lwn.net/ml/oss-security/20221013101046.GB20615@suse.de/

Thanks,
Mostafa

> 
> > - VFIO is something we are looking at the moment and have prototyped with
> >   pKVM, and it should be supported soon in Android (only for platform
> >   devices for now)
> 
> Yes, this makes sense
> 
> Jason