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

Hi Rahul,

On 08/01/2021 14:46, 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>
Aside what Oleksandr pointed out, this code looks ok to me.


Julien Grall



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