Re: [PATCH 3/4] xen: Add DOMAIN_CAPS_DEVICE_MODEL & XEN_DOMCTL_CDF_device_model

[Christian Lindig <christian.lindig@cloud.com>]

Acked-by: Christian Lindig <christian.lindig@cloud.com>

> On 10 Jun 2025, at 23:57, Jason Andryuk <jason.andryuk@amd.com> wrote:
> 
> To add more flexibility in system configuration add the new
> DOMAIN_CAPS_DEVICE_MODEL flag and XEN_DOMCTL_CDF_device_model.
> 
> Thie new flag corresponds to allowing XSM_DM_PRIV for the domain.  This
> will enable running device model emulators (QEMU) from the assigne
> domain for multiple target domains.
> 
> Stubdoms assign target allowing the stubdom to serve as the device
> model for a single domain.  This new flag allows the single domain to
> provide emulators for multiple guests.
> 
> The specific scenario is a disaggregated system with the hardware domain
> providing device models for muitple guest domains.
> 
> The OCaml code needs the flag added in as well.
> 
> Signed-off-by: Jason Andryuk <jason.andryuk@amd.com>
> ---
> tools/ocaml/libs/xc/xenctrl.ml          |  1 +
> tools/ocaml/libs/xc/xenctrl.mli         |  1 +
> xen/arch/arm/domain.c                   |  3 ++-
> xen/common/device-tree/dom0less-build.c |  3 +++
> xen/common/domain.c                     |  3 ++-
> xen/include/public/bootfdt.h            | 12 ++++++++++--
> xen/include/public/domctl.h             |  4 +++-
> xen/include/xen/sched.h                 |  9 +++++++++
> xen/include/xsm/dummy.h                 |  2 ++
> 9 files changed, 33 insertions(+), 5 deletions(-)
> 
> diff --git a/tools/ocaml/libs/xc/xenctrl.ml b/tools/ocaml/libs/xc/xenctrl.ml
> index 2690f9a923..ef0c5dca2a 100644
> --- a/tools/ocaml/libs/xc/xenctrl.ml
> +++ b/tools/ocaml/libs/xc/xenctrl.ml
> @@ -70,6 +70,7 @@ type domain_create_flag =
>   | CDF_IOMMU
>   | CDF_NESTED_VIRT
>   | CDF_VPMU
> +  | CDF_DEVICE_MODEL
> 
> type domain_create_iommu_opts =
>   | IOMMU_NO_SHAREPT
> diff --git a/tools/ocaml/libs/xc/xenctrl.mli b/tools/ocaml/libs/xc/xenctrl.mli
> index febbe1f6ae..c0156fa5c6 100644
> --- a/tools/ocaml/libs/xc/xenctrl.mli
> +++ b/tools/ocaml/libs/xc/xenctrl.mli
> @@ -63,6 +63,7 @@ type domain_create_flag =
>   | CDF_IOMMU
>   | CDF_NESTED_VIRT
>   | CDF_VPMU
> +  | CDF_DEVICE_MODEL
> 
> type domain_create_iommu_opts =
>   | IOMMU_NO_SHAREPT
> diff --git a/xen/arch/arm/domain.c b/xen/arch/arm/domain.c
> index 45aeb8bddc..12fda0762f 100644
> --- a/xen/arch/arm/domain.c
> +++ b/xen/arch/arm/domain.c
> @@ -612,7 +612,8 @@ int arch_sanitise_domain_config(struct xen_domctl_createdomain *config)
>     unsigned int max_vcpus;
>     unsigned int flags_required = (XEN_DOMCTL_CDF_hvm | XEN_DOMCTL_CDF_hap);
>     unsigned int flags_optional = (XEN_DOMCTL_CDF_iommu | XEN_DOMCTL_CDF_vpmu |
> -                                   XEN_DOMCTL_CDF_xs_domain );
> +                                   XEN_DOMCTL_CDF_xs_domain |
> +                                   XEN_DOMCTL_CDF_device_model);
>     unsigned int sve_vl_bits = sve_decode_vl(config->arch.sve_vl);
> 
>     if ( (config->flags & ~flags_optional) != flags_required )
> diff --git a/xen/common/device-tree/dom0less-build.c b/xen/common/device-tree/dom0less-build.c
> index 3d503c6973..993ff32f5e 100644
> --- a/xen/common/device-tree/dom0less-build.c
> +++ b/xen/common/device-tree/dom0less-build.c
> @@ -884,6 +884,9 @@ void __init create_domUs(void)
>                 d_cfg.flags |= XEN_DOMCTL_CDF_xs_domain;
>                 d_cfg.max_evtchn_port = -1;
>             }
> +
> +            if ( val & DOMAIN_CAPS_DEVICE_MODEL )
> +                d_cfg.flags |= XEN_DOMCTL_CDF_device_model;
>         }
> 
>         if ( dt_find_property(node, "xen,static-mem", NULL) )
> diff --git a/xen/common/domain.c b/xen/common/domain.c
> index 153cd75340..ab2c8f864d 100644
> --- a/xen/common/domain.c
> +++ b/xen/common/domain.c
> @@ -721,7 +721,8 @@ static int sanitise_domain_config(struct xen_domctl_createdomain *config)
>          ~(XEN_DOMCTL_CDF_hvm | XEN_DOMCTL_CDF_hap |
>            XEN_DOMCTL_CDF_s3_integrity | XEN_DOMCTL_CDF_oos_off |
>            XEN_DOMCTL_CDF_xs_domain | XEN_DOMCTL_CDF_iommu |
> -           XEN_DOMCTL_CDF_nested_virt | XEN_DOMCTL_CDF_vpmu) )
> +           XEN_DOMCTL_CDF_nested_virt | XEN_DOMCTL_CDF_vpmu |
> +           XEN_DOMCTL_CDF_device_model) )
>     {
>         dprintk(XENLOG_INFO, "Unknown CDF flags %#x\n", config->flags);
>         return -EINVAL;
> diff --git a/xen/include/public/bootfdt.h b/xen/include/public/bootfdt.h
> index 86c46b42a9..c6b5afc76a 100644
> --- a/xen/include/public/bootfdt.h
> +++ b/xen/include/public/bootfdt.h
> @@ -25,7 +25,15 @@
> #define DOMAIN_CAPS_HARDWARE (1U << 1)
> /* Xenstore domain. */
> #define DOMAIN_CAPS_XENSTORE (1U << 2)
> -#define DOMAIN_CAPS_MASK     (DOMAIN_CAPS_CONTROL | DOMAIN_CAPS_HARDWARE | \
> -                              DOMAIN_CAPS_XENSTORE)
> +/*
> + * Device model capability allows the use of the dm_op hypercalls to provide
> + * the device model emulation (run QEMU) for other domains.  This is a
> + * subset of the Control capability which can be granted to the
> + * Hardware domain for running QEMU.
> + */
> +#define DOMAIN_CAPS_DEVICE_MODEL (1U << 3)
> +
> +#define DOMAIN_CAPS_MASK     (DOMAIN_CAPS_CONTROL  | DOMAIN_CAPS_HARDWARE | \
> +                              DOMAIN_CAPS_XENSTORE | DOMAIN_CAPS_DEVICE_MODEL )
> 
> #endif /* __XEN_PUBLIC_BOOTFDT_H__ */
> diff --git a/xen/include/public/domctl.h b/xen/include/public/domctl.h
> index 5b2063eed9..2280489be2 100644
> --- a/xen/include/public/domctl.h
> +++ b/xen/include/public/domctl.h
> @@ -66,9 +66,11 @@ struct xen_domctl_createdomain {
> #define XEN_DOMCTL_CDF_nested_virt    (1U << _XEN_DOMCTL_CDF_nested_virt)
> /* Should we expose the vPMU to the guest? */
> #define XEN_DOMCTL_CDF_vpmu           (1U << 7)
> +/* Allow domain to provide device model for multiple other domains */
> +#define XEN_DOMCTL_CDF_device_model   (1U << 8)
> 
> /* Max XEN_DOMCTL_CDF_* constant.  Used for ABI checking. */
> -#define XEN_DOMCTL_CDF_MAX XEN_DOMCTL_CDF_vpmu
> +#define XEN_DOMCTL_CDF_MAX XEN_DOMCTL_CDF_device_model
> 
>     uint32_t flags;
> 
> diff --git a/xen/include/xen/sched.h b/xen/include/xen/sched.h
> index fe53d4fab7..9863603d93 100644
> --- a/xen/include/xen/sched.h
> +++ b/xen/include/xen/sched.h
> @@ -1148,6 +1148,15 @@ static always_inline bool is_control_domain(const struct domain *d)
>     return evaluate_nospec(d->is_privileged);
> }
> 
> +/* This check is for functionality specific to a device model domain */
> +static always_inline bool is_dm_domain(const struct domain *d)
> +{
> +    if ( IS_ENABLED(CONFIG_PV_SHIM_EXCLUSIVE) )
> +        return false;
> +
> +    return evaluate_nospec(d->options & XEN_DOMCTL_CDF_device_model);
> +}
> +
> #define VM_ASSIST(d, t) (test_bit(VMASST_TYPE_ ## t, &(d)->vm_assist))
> 
> static always_inline bool is_pv_domain(const struct domain *d)
> diff --git a/xen/include/xsm/dummy.h b/xen/include/xsm/dummy.h
> index d8df3f66c4..477fadaefd 100644
> --- a/xen/include/xsm/dummy.h
> +++ b/xen/include/xsm/dummy.h
> @@ -91,6 +91,8 @@ static always_inline int xsm_default_action(
>             return 0;
>         fallthrough;
>     case XSM_DM_PRIV:
> +        if ( is_dm_domain(src) )
> +            return 0;
>         if ( target && evaluate_nospec(src->target == target) )
>             return 0;
>         fallthrough;
> -- 
> 2.49.0
>