Re: [PATCH v2 2/3] intel_iommu: Simplify caching mode check with VFIO device

[CLEMENT MATHIEU--DRIF <clement.mathieu--drif@eviden.com>]

Hi zhenzhong,

Thanks for the respin.  
Just a few comments in the commit message.

On Sun, 2025-09-28 at 23:42 -0400, Zhenzhong Duan wrote:
> In early days, we have different tricks to ensure caching-mode=on with VFIO  

s/have/had

> device:
> 
> 28cf553afe ("intel_iommu: Sanity check vfio-pci config on machine init done")  
> c6cbc29d36 ("pc/q35: Disallow vfio-pci hotplug without VT-d caching mode")
> 
> There is also a patch of same purpose but for VDPA device:

s/of same/with the same

> 
> b8d78277c0 ("intel-iommu: fail MAP notifier without caching mode")
> 
> Because without caching mode, MAP notifier won't work correctly since guest  
> won't send IOTLB update event when it establishes new mappings in the I/O page  
> tables.
> 
> Now with host IOMMU device interface between VFIO and vIOMMU, we can simplify  
> first two commits above with a small check in set_iommu_device(). This also  
> works for future IOMMUFD backed VDPA implementation which may also need caching  
> mode on. But for legacy VDPA we still need commit b8d78277c0 as it doesn't  
> use host IOMMU device interface.

s/use host/use the host

> 
> For coldplug VFIO device:
> 
>   qemu-system-x86_64: -device vfio-pci,host=0000:3b:00.0,id=hostdev3,bus=root0,iommufd=iommufd0: vfio 0000:3b:00.0: Failed to set vIOMMU: Device assignment is not allowed without enabling caching-mode=on for Intel IOMMU.
> 
> For hotplug VFIO device:
> 
>   if "iommu=off" is configured in guest,  
>     Error: vfio 0000:3b:00.0: Failed to set vIOMMU: Device assignment is not allowed without enabling caching-mode=on for Intel IOMMU.  
>   else  
>     Error: vfio 0000:3b:00.0: memory listener initialization failed: Region vtd-00.0-dmar: device 01.00.0 requires caching mode: Operation not supported
> 
> The specialty for hotplug is due to the check in commit b8d78277c0 happen before  
> the check in set_iommu_device.
> 
> Signed-off-by: Zhenzhong Duan <[zhenzhong.duan@intel.com](mailto:zhenzhong.duan@intel.com)>  
> ---  
>  hw/i386/intel_iommu.c | 40 ++++++----------------------------------  
>  hw/i386/pc.c          | 20 --------------------  
>  2 files changed, 6 insertions(+), 54 deletions(-)
> 
> diff --git a/hw/i386/intel_iommu.c b/hw/i386/intel_iommu.c  
> index f04300022e..c634121514 100644  
> --- a/hw/i386/intel_iommu.c  
> +++ b/hw/i386/intel_iommu.c  
> @@ -85,13 +85,6 @@ struct vtd_iotlb_key {  
>  static void vtd_address_space_refresh_all(IntelIOMMUState *s);  
>  static void vtd_address_space_unmap(VTDAddressSpace *as, IOMMUNotifier *n);  
>    
> -static void vtd_panic_require_caching_mode(void)  
> -{  
> -    error_report("We need to set caching-mode=on for intel-iommu to enable "  
> -                 "device assignment with IOMMU protection.");  
> -    exit(1);  
> -}  
> -  
>  static void vtd_define_quad(IntelIOMMUState *s, hwaddr addr, uint64_t val,  
>                              uint64_t wmask, uint64_t w1cmask)  
>  {  
> @@ -4378,6 +4371,12 @@ static bool vtd_dev_set_iommu_device(PCIBus *bus, void *opaque, int devfn,  
>    
>      assert(hiod);  
>    
> +    if (!s->caching_mode) {  
> +        error_setg(errp, "Device assignment is not allowed without enabling "  
> +                   "caching-mode=on for Intel IOMMU.");  
> +        return false;  
> +    }  
> +  
>      vtd_iommu_lock(s);  
>    
>      if (g_hash_table_lookup(s->vtd_host_iommu_dev, &key)) {  
> @@ -4910,32 +4909,6 @@ static bool vtd_decide_config(IntelIOMMUState *s, Error **errp)  
>      return true;  
>  }  
>    
> -static int vtd_machine_done_notify_one(Object *child, void *unused)  
> -{  
> -    IntelIOMMUState *iommu = INTEL_IOMMU_DEVICE(x86_iommu_get_default());  
> -  
> -    /*  
> -     * We hard-coded here because vfio-pci is the only special case  
> -     * here.  Let's be more elegant in the future when we can, but so  
> -     * far there seems to be no better way.  
> -     */  
> -    if (object_dynamic_cast(child, "vfio-pci") && !iommu->caching_mode) {  
> -        vtd_panic_require_caching_mode();  
> -    }  
> -  
> -    return 0;  
> -}  
> -  
> -static void vtd_machine_done_hook(Notifier *notifier, void *unused)  
> -{  
> -    object_child_foreach_recursive(object_get_root(),  
> -                                   vtd_machine_done_notify_one, NULL);  
> -}  
> -  
> -static Notifier vtd_machine_done_notify = {  
> -    .notify = vtd_machine_done_hook,  
> -};  
> -  
>  static void vtd_realize(DeviceState *dev, Error **errp)  
>  {  
>      MachineState *ms = MACHINE(qdev_get_machine());  
> @@ -4990,7 +4963,6 @@ static void vtd_realize(DeviceState *dev, Error **errp)  
>      pci_setup_iommu(bus, &vtd_iommu_ops, dev);  
>      /* Pseudo address space under root PCI bus. */  
>      x86ms->ioapic_as = vtd_host_dma_iommu(bus, s, Q35_PSEUDO_DEVFN_IOAPIC);  
> -    qemu_add_machine_init_done_notifier(&vtd_machine_done_notify);  
>  }  
>    
>  static void vtd_class_init(ObjectClass *klass, const void *data)  
> diff --git a/hw/i386/pc.c b/hw/i386/pc.c  
> index bc048a6d13..01cd9a67db 100644  
> --- a/hw/i386/pc.c  
> +++ b/hw/i386/pc.c  
> @@ -1720,25 +1720,6 @@ static void pc_machine_wakeup(MachineState *machine)  
>      cpu_synchronize_all_post_reset();  
>  }  
>    
> -static bool pc_hotplug_allowed(MachineState *ms, DeviceState *dev, Error **errp)  
> -{  
> -    X86IOMMUState *iommu = x86_iommu_get_default();  
> -    IntelIOMMUState *intel_iommu;  
> -  
> -    if (iommu &&  
> -        object_dynamic_cast((Object *)iommu, TYPE_INTEL_IOMMU_DEVICE) &&  
> -        object_dynamic_cast((Object *)dev, "vfio-pci")) {  
> -        intel_iommu = INTEL_IOMMU_DEVICE(iommu);  
> -        if (!intel_iommu->caching_mode) {  
> -            error_setg(errp, "Device assignment is not allowed without "  
> -                       "enabling caching-mode=on for Intel IOMMU.");  
> -            return false;  
> -        }  
> -    }  
> -  
> -    return true;  
> -}  
> -  
>  static void pc_machine_class_init(ObjectClass *oc, const void *data)  
>  {  
>      MachineClass *mc = MACHINE_CLASS(oc);  
> @@ -1758,7 +1739,6 @@ static void pc_machine_class_init(ObjectClass *oc, const void *data)  
>      x86mc->apic_xrupt_override = true;  
>      assert(!mc->get_hotplug_handler);  
>      mc->get_hotplug_handler = pc_get_hotplug_handler;  
> -    mc->hotplug_allowed = pc_hotplug_allowed;  
>      mc->auto_enable_numa_with_memhp = true;  
>      mc->auto_enable_numa_with_memdev = true;  
>      mc->has_hotpluggable_cpus = true;