Re: [RFC] /dev/ioasid uAPI proposal

[David Gibson]

On Tue, Jun 01, 2021 at 07:09:21PM +0800, Lu Baolu wrote:
> Hi Jason,
> 
> On 2021/5/29 7:36, Jason Gunthorpe wrote:
> > > /*
> > >    * Bind an user-managed I/O page table with the IOMMU
> > >    *
> > >    * Because user page table is untrusted, IOASID nesting must be enabled
> > >    * for this ioasid so the kernel can enforce its DMA isolation policy
> > >    * through the parent ioasid.
> > >    *
> > >    * Pgtable binding protocol is different from DMA mapping. The latter
> > >    * has the I/O page table constructed by the kernel and updated
> > >    * according to user MAP/UNMAP commands. With pgtable binding the
> > >    * whole page table is created and updated by userspace, thus different
> > >    * set of commands are required (bind, iotlb invalidation, page fault, etc.).
> > >    *
> > >    * Because the page table is directly walked by the IOMMU, the user
> > >    * must  use a format compatible to the underlying hardware. It can
> > >    * check the format information through IOASID_GET_INFO.
> > >    *
> > >    * The page table is bound to the IOMMU according to the routing
> > >    * information of each attached device under the specified IOASID. The
> > >    * routing information (RID and optional PASID) is registered when a
> > >    * device is attached to this IOASID through VFIO uAPI.
> > >    *
> > >    * Input parameters:
> > >    *	- child_ioasid;
> > >    *	- address of the user page table;
> > >    *	- formats (vendor, address_width, etc.);
> > >    *
> > >    * Return: 0 on success, -errno on failure.
> > >    */
> > > #define IOASID_BIND_PGTABLE		_IO(IOASID_TYPE, IOASID_BASE + 9)
> > > #define IOASID_UNBIND_PGTABLE	_IO(IOASID_TYPE, IOASID_BASE + 10)
> > Also feels backwards, why wouldn't we specify this, and the required
> > page table format, during alloc time?
> > 
> 
> Thinking of the required page table format, perhaps we should shed more
> light on the page table of an IOASID. So far, an IOASID might represent
> one of the following page tables (might be more):
> 
>  1) an IOMMU format page table (a.k.a. iommu_domain)
>  2) a user application CPU page table (SVA for example)
>  3) a KVM EPT (future option)
>  4) a VM guest managed page table (nesting mode)
> 
> This version only covers 1) and 4). Do you think we need to support 2),

Isn't (2) the equivalent of using the using the host-managed pagetable
then doing a giant MAP of all your user address space into it?  But
maybe we should identify that case explicitly in case the host can
optimize it.

> 3) and beyond? If so, it seems that we need some in-kernel helpers and
> uAPIs to support pre-installing a page table to IOASID. From this point
> of view an IOASID is actually not just a variant of iommu_domain, but an
> I/O page table representation in a broader sense.
> 
> Best regards,
> baolu
> 

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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