Re: [PATCH v8 11/23] intel_iommu_accel: Stick to system MR for IOMMUFD backed host device when x-flts=on

[Eric Auger <eric.auger@redhat.com>]

On 11/17/25 10:37 AM, Zhenzhong Duan wrote:
> When guest enables scalable mode and setup first stage page table, we don't
> want to use IOMMU MR but rather continue using the system MR for IOMMUFD
> backed host device.
>
> Then default HWPT in VFIO contains GPA->HPA mappings which could be reused
> as nesting parent HWPT to construct nested HWPT in vIOMMU.
>
> Move vtd_as_key into intel_iommu_internal.h as it's also used by accel code.
>
> Suggested-by: Yi Liu <yi.l.liu@intel.com>
> Signed-off-by: Zhenzhong Duan <zhenzhong.duan@intel.com>

same here

Reviewed-by: Eric Auger <eric.auger@redhat.com>

Eric
> ---
>  hw/i386/intel_iommu_accel.h    |  6 ++++++
>  hw/i386/intel_iommu_internal.h | 11 +++++++++++
>  hw/i386/intel_iommu.c          | 28 +++++++++++++++-------------
>  hw/i386/intel_iommu_accel.c    | 18 ++++++++++++++++++
>  4 files changed, 50 insertions(+), 13 deletions(-)
>
> diff --git a/hw/i386/intel_iommu_accel.h b/hw/i386/intel_iommu_accel.h
> index 7ebf137a1a..dbe6ee6982 100644
> --- a/hw/i386/intel_iommu_accel.h
> +++ b/hw/i386/intel_iommu_accel.h
> @@ -15,6 +15,7 @@
>  #ifdef CONFIG_VTD_ACCEL
>  bool vtd_check_hiod_accel(IntelIOMMUState *s, VTDHostIOMMUDevice *vtd_hiod,
>                            Error **errp);
> +VTDHostIOMMUDevice *vtd_find_hiod_iommufd(VTDAddressSpace *as);
>  #else
>  static inline bool vtd_check_hiod_accel(IntelIOMMUState *s,
>                                          VTDHostIOMMUDevice *vtd_hiod,
> @@ -24,5 +25,10 @@ static inline bool vtd_check_hiod_accel(IntelIOMMUState *s,
>                 "host IOMMU is incompatible with guest first stage translation");
>      return false;
>  }
> +
> +static inline VTDHostIOMMUDevice *vtd_find_hiod_iommufd(VTDAddressSpace *as)
> +{
> +    return NULL;
> +}
>  #endif
>  #endif
> diff --git a/hw/i386/intel_iommu_internal.h b/hw/i386/intel_iommu_internal.h
> index 02522f64e0..d8dad18304 100644
> --- a/hw/i386/intel_iommu_internal.h
> +++ b/hw/i386/intel_iommu_internal.h
> @@ -685,4 +685,15 @@ typedef struct VTDHostIOMMUDevice {
>      uint8_t devfn;
>      HostIOMMUDevice *hiod;
>  } VTDHostIOMMUDevice;
> +
> +/*
> + * PCI bus number (or SID) is not reliable since the device is usaully
> + * initialized before guest can configure the PCI bridge
> + * (SECONDARY_BUS_NUMBER).
> + */
> +struct vtd_as_key {
> +    PCIBus *bus;
> +    uint8_t devfn;
> +    uint32_t pasid;
> +};
>  #endif
> diff --git a/hw/i386/intel_iommu.c b/hw/i386/intel_iommu.c
> index 4ebf56a74f..29e0281af8 100644
> --- a/hw/i386/intel_iommu.c
> +++ b/hw/i386/intel_iommu.c
> @@ -56,17 +56,6 @@
>  #define VTD_PE_GET_SS_LEVEL(pe) \
>      (2 + (((pe)->val[0] >> 2) & VTD_SM_PASID_ENTRY_AW))
>  
> -/*
> - * PCI bus number (or SID) is not reliable since the device is usaully
> - * initialized before guest can configure the PCI bridge
> - * (SECONDARY_BUS_NUMBER).
> - */
> -struct vtd_as_key {
> -    PCIBus *bus;
> -    uint8_t devfn;
> -    uint32_t pasid;
> -};
> -
>  /* bus/devfn is PCI device's real BDF not the aliased one */
>  struct vtd_hiod_key {
>      PCIBus *bus;
> @@ -1731,12 +1720,25 @@ static bool vtd_as_pt_enabled(VTDAddressSpace *as)
>  /* Return whether the device is using IOMMU translation. */
>  static bool vtd_switch_address_space(VTDAddressSpace *as)
>  {
> +    IntelIOMMUState *s;
>      bool use_iommu, pt;
>  
>      assert(as);
>  
> -    use_iommu = as->iommu_state->dmar_enabled && !vtd_as_pt_enabled(as);
> -    pt = as->iommu_state->dmar_enabled && vtd_as_pt_enabled(as);
> +    s = as->iommu_state;
> +    use_iommu = s->dmar_enabled && !vtd_as_pt_enabled(as);
> +    pt = s->dmar_enabled && vtd_as_pt_enabled(as);
> +
> +    /*
> +     * When guest enables scalable mode and sets up first stage page table,
> +     * we stick to system MR for IOMMUFD backed host device. Then its
> +     * default hwpt contains GPA->HPA mappings which is used directly if
> +     * PGTT=PT and used as nesting parent if PGTT=FST. Otherwise fall back
> +     * to original processing.
> +     */
> +    if (s->root_scalable && s->fsts && vtd_find_hiod_iommufd(as)) {
> +        use_iommu = false;
> +    }
>  
>      trace_vtd_switch_address_space(pci_bus_num(as->bus),
>                                     VTD_PCI_SLOT(as->devfn),
> diff --git a/hw/i386/intel_iommu_accel.c b/hw/i386/intel_iommu_accel.c
> index ead6c42879..ebfc503d64 100644
> --- a/hw/i386/intel_iommu_accel.c
> +++ b/hw/i386/intel_iommu_accel.c
> @@ -50,3 +50,21 @@ bool vtd_check_hiod_accel(IntelIOMMUState *s, VTDHostIOMMUDevice *vtd_hiod,
>                 "host IOMMU is incompatible with guest first stage translation");
>      return false;
>  }
> +
> +VTDHostIOMMUDevice *vtd_find_hiod_iommufd(VTDAddressSpace *as)
> +{
> +    IntelIOMMUState *s = as->iommu_state;
> +    struct vtd_as_key key = {
> +        .bus = as->bus,
> +        .devfn = as->devfn,
> +    };
> +    VTDHostIOMMUDevice *vtd_hiod = g_hash_table_lookup(s->vtd_host_iommu_dev,
> +                                                       &key);
> +
> +    if (vtd_hiod && vtd_hiod->hiod &&
> +        object_dynamic_cast(OBJECT(vtd_hiod->hiod),
> +                            TYPE_HOST_IOMMU_DEVICE_IOMMUFD)) {
> +        return vtd_hiod;
> +    }
> +    return NULL;
> +}