Re: [PATCH v4 11/11] xen/arm: smmuv3: Add support for SMMUv3 driver

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Fri, 8 Jan 2021, Rahul Singh wrote:
> Add support for ARM architected SMMUv3 implementation. It is based on
> the Linux SMMUv3 driver.
> 
> Driver is currently supported as Tech Preview.
> 
> Major differences with regard to Linux driver are as follows:
> 2. Only Stage-2 translation is supported as compared to the Linux driver
>    that supports both Stage-1 and Stage-2 translations.
> 3. Use P2M  page table instead of creating one as SMMUv3 has the
>    capability to share the page tables with the CPU.
> 4. Tasklets are used in place of threaded IRQ's in Linux for event queue
>    and priority queue IRQ handling.
> 5. Latest version of the Linux SMMUv3 code implements the commands queue
>    access functions based on atomic operations implemented in Linux.
>    Atomic functions used by the commands queue access functions are not
>    implemented in XEN therefore we decided to port the earlier version
>    of the code. Atomic operations are introduced to fix the bottleneck
>    of the SMMU command queue insertion operation. A new algorithm for
>    inserting commands into the queue is introduced, which is lock-free
>    on the fast-path.
>    Consequence of reverting the patch is that the command queue
>    insertion will be slow for large systems as spinlock will be used to
>    serializes accesses from all CPUs to the single queue supported by
>    the hardware. Once the proper atomic operations will be available in
>    XEN the driver can be updated.
> 6. Spin lock is used in place of mutex when attaching a device to the
>    SMMU, as there is no blocking locks implementation available in XEN.
>    This might introduce latency in XEN. Need to investigate before
>    driver is out for tech preview.
> 7. PCI ATS functionality is not supported, as there is no support
>    available in XEN to test the functionality. Code is not tested and
>    compiled. Code is guarded by the flag CONFIG_PCI_ATS.
> 8. MSI interrupts are not supported as there is no support available in
>    XEN to request MSI interrupts. Code is not tested and compiled. Code
>    is guarded by the flag CONFIG_MSI.
>
> Signed-off-by: Rahul Singh <rahul.singh@xxxxxxx>

Going through the code you addressed all my previous comments and I
don't have any to add, so:

Acked-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>