RE: Support SVM without PASID

["Tian, Kevin" <kevin.tian@intel.com>]

> From: Bharat Kumar Gogada [mailto:bharatku@xilinx.com]
> Sent: Monday, August 28, 2017 9:10 PM
> 
> > Subject: RE: Support SVM without PASID
> >
> > > From: valmiki [mailto:valmikibow@gmail.com]
> > > Sent: Saturday, August 12, 2017 8:11 PM
> > >
> > > On 8/7/2017 4:01 PM, Jean-Philippe Brucker wrote:
> > > > On 05/08/17 06:14, valmiki wrote:
> > > > [...]
> > > >> Hi Jean, Thanks a lot, now i understood the flow. From vfio kernel
> > > >> documentation we fill vaddr and iova in struct
> > > vfio_iommu_type1_dma_map
> > > >> and pass them to VFIO. But if we use dynamic allocation in
> > > >> application (say malloc), do we need to use dma API to get iova and
> > > >> then call VFIO_IOMMU_MAP ioctl ?
> > > >> If application needs multiple such dynamic allocations, then it
> > > >> need to allocate large chunk and program it via VFIO_IOMMU_MAP
> > > >> ioctl and
> > > then
> > > >> manage rest allocations requirements from this buffer ?
> > > >
> > > > Yes, without SVM, the application allocates large buffers, allocates
> > > > IOVAs itself, and maps them with VFIO_IOMMU_MAP. Userspace
> doesn't
> > > > rely
> > > on the
> > > > DMA API at all, it manages IOVAs as it wants. Sizes passed to
> > > > VFIO_IOMMU_MAP have to be multiples of the MMU or IOMMU page
> > > granularity
> > > > (that is at least 4kB), and both iova and vaddr have to be aligned
> > > > on that granularity as well. So malloc isn't really suitable in this
> > > > case, you'll need mmap. The application can then implement a small
> > > > allocator to
> > > manage
> > > > the DMA pool created with VFIO_IOMMU_MAP.
> > >
> > > Thanks Jean, I have a confusion allocate IOVA's in userspace means,
> > > how can user application decide IOVA address, can user application
> > > pick any random IOVA address ?
> > >
> >
> > yes, any address. In this mode the whole IOVA address space is owned by
> > application, which just needs to use VFIO_IOMMU_MAP to setup
> > IOVA->PA mapping in IOMMU (As Jean pointed out, input paramters
> > are iova and vaddr. VFIO will figure out pa corresponding to vaddr).
> >
> Hi Kevin, I have a doubt in this case, what if someone assigns mmap
> returned virtual address as iova address, then
> EP will assume it is generating request on IOVA but in reality we are using
> application allocated virtual address, then
> it looks like we are working with application virtual address directly without
> PASID. Is this valid ?
> 

IOMMU doesn't care what an address in the DMA transaction from
a device is. It cares only whether the DMA transaction has PASID
tagged or not, and then pursue different structure to walk corresponding 
I/O page table to get a translated address. Each I/O page table hosts
a standalone virtual address space. An application can implement a
new allocator to manage the address space (iova != vaddr), or simply
reuse malloc-ed virtual address (iova == vaddr), which is not what
hardware really cares about.

Thanks
Kevin