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

[Jason Gunthorpe]

On Mon, Oct 04, 2021 at 09:40:03AM -0700, Jacob Pan wrote:
> Hi Barry,
> 
> On Sat, 2 Oct 2021 01:45:59 +1300, Barry Song <21cnbao@xxxxxxxxx> 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.

The IOTLB flush is not on a DMA path but on a vmap path, so it is very
hard to compare the two things.. Maybe vmap can be made to do lazy
IOTLB flush or something and it could be closer

> Security-wise, KVA respects kernel mapping. So permissions are better
> enforced than pass-through and identity mapping.

Is this meaningful? Isn't the entire physical map still in the KVA and
isn't it entirely RW ?

Jason