Re: [RFC 0/7] Support in-kernel DMA with PASID and SVA

Jacob Pan
2021-10-04T18:19:02Z

Hi Barry,

On Sat, 2 Oct 2021 01:45:59 +1300, Barry Song <21cnbao@gmail.com> wrote:

> >
> > > I assume KVA mode can avoid this iotlb flush as the device is using
> > > the page table of the kernel and sharing the whole kernel space. But
> > > will users be glad to accept this mode?
> >
> > You can avoid the lock be identity mapping the physical address space
> > of the kernel and maping map/unmap a NOP.
> >
> > KVA is just a different way to achive this identity map with slightly
> > different security properties than the normal way, but it doesn't
> > reach to the same security level as proper map/unmap.
> >
> > I'm not sure anyone who cares about DMA security would see value in
> > the slight difference between KVA and a normal identity map.
>
> yes. This is an important question. if users want a high security level,
> kva might not their choice; if users don't want the security, they are
> using iommu passthrough. So when will users choose KVA?
Right, KVAs sit in the middle in terms of performance and security.
Performance is better than IOVA due to IOTLB flush as you mentioned. Also
not too far behind of pass-through.

Security-wise, KVA respects kernel mapping. So permissions are better
enforced than pass-through and identity mapping.
To balance performance and security, we are proposing KVA is only supported
on trusted devices. On an Intel platform, it would be based on ACPI SATC
(SoC Integrated Address Translation Cache (SATC) reporting structure, VT-d
spec. 8.2). I am also adding a kernel iommu parameter to allow user
override.

Thanks,

Jacob