Re: [RFC 11/20] iommu/iommufd: Add IOMMU_IOASID_ALLOC/FREE

[david@xxxxxxxxxxxxxxxxxxxxx]

On Wed, Sep 22, 2021 at 11:09:11AM -0300, Jason Gunthorpe wrote:
> On Wed, Sep 22, 2021 at 03:40:25AM +0000, Tian, Kevin wrote:
> > > From: Jason Gunthorpe <jgg@xxxxxxxxxx>
> > > Sent: Wednesday, September 22, 2021 1:45 AM
> > > 
> > > On Sun, Sep 19, 2021 at 02:38:39PM +0800, Liu Yi L wrote:
> > > > This patch adds IOASID allocation/free interface per iommufd. When
> > > > allocating an IOASID, userspace is expected to specify the type and
> > > > format information for the target I/O page table.
> > > >
> > > > This RFC supports only one type (IOMMU_IOASID_TYPE_KERNEL_TYPE1V2),
> > > > implying a kernel-managed I/O page table with vfio type1v2 mapping
> > > > semantics. For this type the user should specify the addr_width of
> > > > the I/O address space and whether the I/O page table is created in
> > > > an iommu enfore_snoop format. enforce_snoop must be true at this point,
> > > > as the false setting requires additional contract with KVM on handling
> > > > WBINVD emulation, which can be added later.
> > > >
> > > > Userspace is expected to call IOMMU_CHECK_EXTENSION (see next patch)
> > > > for what formats can be specified when allocating an IOASID.
> > > >
> > > > Open:
> > > > - Devices on PPC platform currently use a different iommu driver in vfio.
> > > >   Per previous discussion they can also use vfio type1v2 as long as there
> > > >   is a way to claim a specific iova range from a system-wide address space.
> > > >   This requirement doesn't sound PPC specific, as addr_width for pci
> > > devices
> > > >   can be also represented by a range [0, 2^addr_width-1]. This RFC hasn't
> > > >   adopted this design yet. We hope to have formal alignment in v1
> > > discussion
> > > >   and then decide how to incorporate it in v2.
> > > 
> > > I think the request was to include a start/end IO address hint when
> > > creating the ios. When the kernel creates it then it can return the
> > 
> > is the hint single-range or could be multiple-ranges?
> 
> David explained it here:
> 
> https://lore.kernel.org/kvm/YMrKksUeNW%2FPEGPM@yekko/

Apparently not well enough.  I've attempted again in this thread.

> qeumu needs to be able to chooose if it gets the 32 bit range or 64
> bit range.

No. qemu needs to supply *both* the 32-bit and 64-bit range to its
guest, and therefore needs to request both from the host.

Or rather, it *might* need to supply both.  It will supply just the
32-bit range by default, but the guest can request the 64-bit range
and/or remove and resize the 32-bit range via hypercall interfaces.
Vaguely recent Linux guests certainly will request the 64-bit range in
addition to the default 32-bit range.

-- 
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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