Re: [RFC PATCH] iommu/arm-smmu-v3: Allow nested attach for PCI bridges without vDEVICE

[Pranjal Shrivastava <praan@google.com>]

On Fri, Aug 14, 2026 at 10:07:09AM +0000, Dmitry Malkin wrote:

Hi Dmitry,

[...]

> ---
>  drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3-iommufd.c | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
> 
> diff --git a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3-iommufd.c b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3-iommufd.c
> --- a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3-iommufd.c
> +++ b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3-iommufd.c
> @@ -5,6 +5,8 @@
>  
>  #include <uapi/linux/iommufd.h>
>  
> +#include <linux/pci.h>
> +
>  #include "arm-smmu-v3.h"
>  
>  void *arm_smmu_hw_info(struct device *dev, u32 *length,
> @@ -118,6 +120,17 @@ int arm_smmu_attach_prepare_vmaster(struct arm_smmu_attach_state *state,
>  		if (cfg == STRTAB_STE_0_CFG_ABORT ||
>  		    cfg == STRTAB_STE_0_CFG_BYPASS)
>  			return 0;
> +		/*
> +		 * Group-wide domain attachment also visits host PCI bridges. Such a
> +		 * bridge is not exposed to the VM and therefore has no virtual SID.
> +		 */
> +		if (dev_is_pci(state->master->dev) &&
> +		    pci_is_bridge(to_pci_dev(state->master->dev))) {
> +			dev_info_ratelimited(state->master->dev,
> +					     "skipping vDEVICE requirement for translated nested domain: cfg=%u ret=%d\n",
> +					     cfg, ret);
> +			return 0;
> +		}
>  		return ret;
>  	}


I agree that this is needed for old bridges. However, should this check
actually live higher up in IOMMUFD? Instead of the SMMU driver deciding
to bypass the error, what if iommufd itself intercepts the vDEVICE 
mapping failure during the group iteration? If IOMMUFD sees that the 
device lacking a vDEVICE is an IOMMU group alias/bridge, it could 
explicitly tell the underlying driver to proceed with a NULL vmaster?

Also, regarding RID aliasing: while it's true the host bridge doesn't 
need a vDEVICE for its own host-consumed DMAs (like AER/PME), are we
confident this won't break guest-injected events if the bridge aliases
the downstream endpoint's traffic? For e.g. if the "real" endpoint's
traffic is aliased to the bridge's RID and the bridge has no vmaster,
won't we lose the ability to inject IO Page faults into the guest?

Thanks,
Praan