Re: [RFC] /dev/ioasid uAPI proposal

[Lu Baolu]

Hi David,

On 6/3/21 1:54 PM, David Gibson wrote:
> 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.

Conceptually, yes. Current SVA implementation just reuses the
application's cpu page table w/o map/unmap operations.

Best regards,
baolu