Re: [Xen-devel] [PATCH V2 1/25] DOMCTL: Introduce new DOMCTL commands for vIOMMU support

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Wed, Aug 23, 2017 at 11:19:01AM +0100, Julien Grall wrote:
> Hi Roger,
> 
> On 23/08/17 08:22, Roger Pau Monné wrote:
> > On Wed, Aug 23, 2017 at 02:06:17PM +0800, Lan Tianyu wrote:
> > > Hi Roger:
> > >   Thanks for your review.
> > > 
> > > On 2017年08月22日 22:32, Roger Pau Monné wrote:
> > > > On Wed, Aug 09, 2017 at 04:34:02PM -0400, Lan Tianyu wrote:
> > > > > +
> > > > > +/* vIOMMU capabilities */
> > > > > +#define VIOMMU_CAP_IRQ_REMAPPING  (1u << 0)
> > > > > +
> > > > > +struct xen_domctl_viommu_op {
> > > > > +    uint32_t cmd;
> > > > > +#define XEN_DOMCTL_create_viommu          0
> > > > > +#define XEN_DOMCTL_destroy_viommu         1
> > > > > +#define XEN_DOMCTL_query_viommu_caps      2
> > > > > +    union {
> > > > > +        struct {
> > > > > +            /* IN - vIOMMU type */
> > > > > +            uint64_t viommu_type;
> > > > > +            /*
> > > > > +             * IN - MMIO base address of vIOMMU. vIOMMU device models
> > > > > +             * are in charge of to check base_address and length.
> > > > > +             */
> > > > > +            uint64_t base_address;
> > > > > +            /* IN - Length of MMIO region */
> > > > > +            uint64_t length;
> > > > 
> > > > It seems weird that you can specify the length, is that something
> > > > that a user would like to set? Isn't the length of the IOMMU MMIO
> > > > region fixed by the hardware spec?
> > > 
> > > Different vendor may have different IOMMU register region sizes. (e.g,
> > > VTD has one page size for register region). The length field is to make
> > > vIOMMU device model not to abuse address space. Some registers' offsets
> > > are reported by other register and these offsets are emulated by vIOMMU
> > > device model. If it's not necessary, we can remove it and add it when
> > > there is real such requirement.
> > 
> > So from my understanding the size of the IOMMU MMIO region is implicit
> > in the IOMMU type that the user chooses. I don't think this field is
> > needed.
> 
> To me, it makes more sense to care both the base and the size rather than
> only the former.
> 
> The toolstack is in charge of the address space and should be aware of the
> size of everything. This address space may not be static and it makes sense
> to give this information to Xen and verify we had the same assumption.

Does this imply that we will have variable size vIOMMU MMIO regions?

If not the toolstack should know the size of the MMIO region at all
times, unless you are running a toolstack version != Xen version,
which is not supported.

> > 
> > > > 
> > > > > +            /* IN - Capabilities with which we want to create */
> > > > > +            uint64_t capabilities;
> > > > > +            /* OUT - vIOMMU identity */
> > > > > +            uint32_t viommu_id;
> > > > > +        } create_viommu;
> > > > > +
> > > > > +        struct {
> > > > > +            /* IN - vIOMMU identity */
> > > > > +            uint32_t viommu_id;
> > > > > +        } destroy_viommu;
> > > > > +
> > > > > +        struct {
> > > > > +            /* IN - vIOMMU type */
> > > > > +            uint64_t viommu_type;
> > > > > +            /* OUT - vIOMMU Capabilities */
> > > > > +            uint64_t capabilities;
> > > > > +        } query_caps;
> > > > 
> > > > This also seems weird, shouldn't you query the capabilities of an
> > > > already created vIOMMU, rather than a vIOMMU type? Shouldn't the first
> > > > field be viommu_id?
> > > > 
> > > 
> > > Query interface here is to check what capabilities the vIOMMU device
> > > model specified by viommu_type can support before create vIOMMU (suppose
> > > user may select different capabilities). If capabilities returned by
> > > query interface doesn't meet user configuration, tool stack should
> > > return error. So it just accepts viommu_type.
> > 
> > I don't think that's needed, if the chosen capabilities are not
> > supported by the selected IOMMU type simply return error in
> > XEN_DOMCTL_create_viommu.
> > 
> > The capabilities of each IOMMU type should be listed in the man page,
> > and the user should select a supported set or else
> > XEN_DOMCTL_create_viommu will fail. Doing the checks both in the
> > toolstack and in XEN_DOMCTL_create_viommu seems pointless and prone to
> > errors.
> 
> What if the some capabilities depends on host IOMMU? How are you going to
> report that to the user?

I would print a message on the hypervisor console, I don't see the
value of doing the same check in the toolstack that Xen will also need
to do in XEN_DOMCTL_create_viommu.

I would see value on having such a query hypercall once we have an
implementation that indeed has different capabilities depending on the
hardware, and once a xl command to fetch and print such capabilities
is introduced.

As said, the above query is only used to perform the checks done in
XEN_DOMCTL_create_viommu on the toolstack.

Roger.

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