When vIOMMU is configured x-flts=on in scalable mode, first stage page table
is passed to host to construct nested page table for passthrough devices.

We need to check compatibility of some critical IOMMU capabilities between
vIOMMU and host IOMMU to ensure guest first stage page table could be used by
host.

For instance, vIOMMU supports first stage 1GB large page mapping, but host does
not, then this IOMMUFD backed device should fail.

Even of the checks pass, for now we willingly reject the association because
all the bits are not there yet, it will be relaxed in the end of this series.

Reviewed-by: Yi Liu <yi.l.liu@intel.com>
Signed-off-by: Zhenzhong Duan <zhenzhong.duan@intel.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
---
 hw/i386/intel_iommu.c | 25 ++++++++++++++++++++++++-
 1 file changed, 24 insertions(+), 1 deletion(-)

diff --git a/hw/i386/intel_iommu.c b/hw/i386/intel_iommu.c
index bd80de1670..bcfbc5dd46 100644
--- a/hw/i386/intel_iommu.c
+++ b/hw/i386/intel_iommu.c
@@ -4387,8 +4387,31 @@ static bool vtd_check_hiod(IntelIOMMUState *s, HostIOMMUDevice *hiod,
         return true;
     }
 
+#ifdef CONFIG_IOMMUFD
+    struct HostIOMMUDeviceCaps *caps = &hiod->caps;
+    struct iommu_hw_info_vtd *vtd = &caps->vendor_caps.vtd;
+
+    /* Remaining checks are all first stage translation specific */
+    if (!object_dynamic_cast(OBJECT(hiod), TYPE_HOST_IOMMU_DEVICE_IOMMUFD)) {
+        error_setg(errp, "Need IOMMUFD backend when x-flts=on");
+        return false;
+    }
+
+    if (caps->type != IOMMU_HW_INFO_TYPE_INTEL_VTD) {
+        error_setg(errp, "Incompatible host platform IOMMU type %d",
+                   caps->type);
+        return false;
+    }
+
+    if (s->fs1gp && !(vtd->cap_reg & VTD_CAP_FS1GP)) {
+        error_setg(errp,
+                   "First stage 1GB large page is unsupported by host IOMMU");
+        return false;
+    }
+#endif
+
     error_setg(errp,
-               "host device is uncompatible with first stage translation");
+               "host IOMMU is incompatible with guest first stage translation");
     return false;
 }
 
-- 
2.47.1

On 2025/9/18 16:57, Zhenzhong Duan wrote:
> When vIOMMU is configured x-flts=on in scalable mode, first stage page table
> is passed to host to construct nested page table for passthrough devices.
> 
> We need to check compatibility of some critical IOMMU capabilities between
> vIOMMU and host IOMMU to ensure guest first stage page table could be used by
> host.
> 
> For instance, vIOMMU supports first stage 1GB large page mapping, but host does
> not, then this IOMMUFD backed device should fail.
> 
> Even of the checks pass, for now we willingly reject the association because
> all the bits are not there yet, it will be relaxed in the end of this series.

might be good to note that nested cap is required but it's already
covered in the core, so this patch does not check it. Otherwise, it
would be a question why it is not added without checking previous comments.

Regards,
Yi Liu

>-----Original Message-----
>From: Liu, Yi L <yi.l.liu@intel.com>
>Subject: Re: [PATCH v6 10/22] intel_iommu: Check for compatibility with
>IOMMUFD backed device when x-flts=on
>
>On 2025/9/18 16:57, Zhenzhong Duan wrote:
>> When vIOMMU is configured x-flts=on in scalable mode, first stage page
>table
>> is passed to host to construct nested page table for passthrough devices.
>>
>> We need to check compatibility of some critical IOMMU capabilities
>between
>> vIOMMU and host IOMMU to ensure guest first stage page table could be
>used by
>> host.
>>
>> For instance, vIOMMU supports first stage 1GB large page mapping, but
>host does
>> not, then this IOMMUFD backed device should fail.
>>
>> Even of the checks pass, for now we willingly reject the association because
>> all the bits are not there yet, it will be relaxed in the end of this series.
>
>might be good to note that nested cap is required but it's already
>covered in the core, so this patch does not check it. Otherwise, it
>would be a question why it is not added without checking previous comments.

Sure, will add:

"Note vIOMMU has exposed IOMMU_HWPT_ALLOC_NEST_PARENT flag to force VFIO core to
create nesting parent HWPT, if host doesn't support nested translation, the
creation will fail. So no need to check nested capability here."

Thanks
Zhenzhong