Re: [RFC PATCH 2/6] drivers core: Add I/O ASID allocator

["Raj, Ashok" <ashok.raj-VuQAYsv1563Yd54FQh9/CA@public.gmane.org>]

On Mon, Oct 22, 2018 at 12:49:47PM +0800, Lu Baolu wrote:
> Hi,
> 
> On 10/20/18 2:11 AM, Jean-Philippe Brucker wrote:
> > Some devices might support multiple DMA address spaces, in particular
> > those that have the PCI PASID feature. PASID (Process Address Space ID)
> > allows to share process address spaces with devices (SVA), partition a
> > device into VM-assignable entities (VFIO mdev) or simply provide
> > multiple DMA address space to kernel drivers. Add a global PASID
> > allocator usable by different drivers at the same time. Name it I/O ASID
> > to avoid confusion with ASIDs allocated by arch code, which are usually
> > a separate ID space.
> > 
> > The IOASID space is global. Each device can have its own PASID space,
> > but by convention the IOMMU ended up having a global PASID space, so
> > that with SVA, each mm_struct is associated to a single PASID.
> > 
> > The allocator doesn't really belong in drivers/iommu because some
> > drivers would like to allocate PASIDs for devices that aren't managed by
> > an IOMMU, using the same ID space as IOMMU. It doesn't really belong in
> > drivers/pci either since platform device also support PASID. Add the
> > allocator in drivers/base.
> 
> One concern of moving pasid allocator here is about paravirtual
> allocation of pasid.
> 
> Since there is only a single set of pasid tables which is controlled by

Minor correction: Single system wide PASID namespace, but PASID tables
would be created ideally per-bdf for isolation purposes. 

I'm sure you meant name space, but didn't want that to be mis-interpreted.


> the host, the pasid is a system wide resource. When a driver running in
> a guest VM wants to consume a pasid, it must be intercepted by the
> simulation software and routed the allocation to the host via VFIO. Some
> iommu arch's provide mechanisms to aid this, for example, the virtual
> command interfaces defined in vt-d 3.0. Any pasid used in guest VM
> should go through the virtual command interfaces.
> 

Cheers,
Ashok