Re: [PATCH v6 09/22] intel_iommu: Stick to system MR for IOMMUFD backed host device when x-fls=on

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

Hi Zhenzhong,

On 9/18/25 10:57 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.
>
> Suggested-by: Yi Liu <yi.l.liu@intel.com>
> Signed-off-by: Zhenzhong Duan <zhenzhong.duan@intel.com>
> ---
>  hw/i386/intel_iommu.c | 37 +++++++++++++++++++++++++++++++++++--
>  1 file changed, 35 insertions(+), 2 deletions(-)
>
> diff --git a/hw/i386/intel_iommu.c b/hw/i386/intel_iommu.c
> index ba40649c85..bd80de1670 100644
> --- a/hw/i386/intel_iommu.c
> +++ b/hw/i386/intel_iommu.c
> @@ -40,6 +40,7 @@
>  #include "kvm/kvm_i386.h"
>  #include "migration/vmstate.h"
>  #include "trace.h"
> +#include "system/iommufd.h"
>  
>  /* context entry operations */
>  #define RID_PASID    0
> @@ -1702,6 +1703,24 @@ static bool vtd_dev_pt_enabled(IntelIOMMUState *s, VTDContextEntry *ce,
>  
>  }
>  
> +static 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;
> +}
> +
>  static bool vtd_as_pt_enabled(VTDAddressSpace *as)
>  {
>      IntelIOMMUState *s;
> @@ -1710,6 +1729,7 @@ static bool vtd_as_pt_enabled(VTDAddressSpace *as)
>      assert(as);
>  
>      s = as->iommu_state;
> +
not needed
>      if (vtd_dev_to_context_entry(s, pci_bus_num(as->bus), as->devfn,
>                                   &ce)) {
>          /*
> @@ -1727,12 +1747,25 @@ 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;
nit: init could be done at declaration
> +    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 setup first stage page table,
sets up?
> +     * 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
> +     * fallback to original processing.
fall back?
> +     */
> +    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),
Besides
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Eric