Messages in this thread | | | From | Barry Song <> | Date | Thu, 7 Oct 2021 18:43:33 +1300 | Subject | Re: [RFC 0/7] Support in-kernel DMA with PASID and SVA |
| |
On Tue, Oct 5, 2021 at 7:21 AM Jason Gunthorpe <jgg@nvidia.com> wrote: > > 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@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. > > 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 ?
Some areas are RX, for example, ARCH64 supports KERNEL_TEXT_RDONLY. But the difference is really minor.
So do we have a case where devices can directly access the kernel's data structure such as a list/graph/tree with pointers to a kernel virtual address? then devices don't need to translate the address of pointers in a structure. I assume this is one of the most useful features userspace SVA can provide.
But do we have a case where accelerators/GPU want to use the complex data structures of kernel drivers?
> > Jason
Thanks barry
| |