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

Barry Song
2021-10-01T14:20:41Z

On Sat, Oct 2, 2021 at 1:36 AM Jason Gunthorpe <jgg@nvidia.com> wrote:
>
> On Sat, Oct 02, 2021 at 01:24:54AM +1300, Barry Song 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?

>
> > which have been mapped in the current dma-map/unmap with IOMMU backend.
> > some drivers are using bouncing buffer to overcome the performance loss of
> > dma_map/unmap as copying is faster than unmapping:
> > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=907676b130711fd1f
>
> It is pretty unforuntate that drivers are hard coding behaviors based
> on assumptions of what the portable API is doing under the covers.

not real when it has a tx_copybreak which can be set by ethtool or
similar userspace
tools . if users are using iommu passthrough, copying won't happen by
the default
tx_copybreak. if users are using restrict iommu mode, socket buffers
are copied into
the buffers allocated and mapped in the driver. so this won't require
mapping and
unmapping socket buffers frequently.

>
> Jason

Thanks
barry