Can register addresses conversion go through iommu too? probably not..

[Mulyadi Santosa]

Correct

It is done automatically with the help of so called shadow page table

On Thu, Sep 30, 2021, 13:54 <ckim at etri.re.kr> wrote:

> Hello all,
>
> I know when a device does dma to memory, we use functions like
> dma_alloc_coherent to allocate a memory region for the dma and we get the
> kernel virtual address and device virtual address at the same time. The
> iommu hardware converts the device virtual address to physical address
> reducing the processing time.
>
>
>
> I have a question (to make sure my understanding is correct).
>
> To use registers in the device driver, we use ioremap function to get the
> kernel virtual address for the device registers. If the OS is running in
> virtualized environment (a hypervisor under it), this kernel virtual
> address should go under conversion twice
>
> 1. virtual to physical by OS  (this physical is not actually physical, an
> intermediate physical?)
>
> 2. intermediate physical to real physical
>
> I thought these two conversions are done by kernel or hypervisor software
> because iommu is for address translation for the addresses that the device
> “uses(or generates)”, not for the kernel to access the device itself.
>
> Is my understanding correct?
>
> Any help will be appreciated.
>
> Thank you!
>
> Chan Kim
>
>
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