SMMU: Why do the software in EL1 can modify the TTB of S2 in stream table entry when stage 2 is enabled?

Hi, 

but it is usually not controlled by the OS kernel running in EL1

>>>Why?  In my opinion, the S1 information of the device should be clear to kernel.

Yes, if so, it's safe.

But the hypervisor might need to somehow inform the VM that different streamid corresponds to different CD addresses?

Hi, 

but it is usually not controlled by the OS kernel running in EL1

>>>Why?  In my opinion, the S1 information of the device should be clear to kernel.

Yes, if so, it's safe.

But the hypervisor might need to somehow inform the VM that different streamid corresponds to different CD addresses?

Usually one streamid corresponds to one device. When the hypervisor assigns a device to a VM it will setup the STE.

For stage 1 handling delegation to the guest a viommu is necessary. When the guest programs the CD address in the viommu, the vmm will relay the request to the hypervisor using e.g. a vfio ioctl. See this patch series for an example: https://lkml.org/lkml/2021/2/23/792

Got it, thanks a lot!