Re: Device address specific mapping of arm,mmu-500

[Ray Jui]

Hi Marc/Robin/Will,

On 5/30/17 10:27 AM, Marc Zyngier wrote:
> On 30/05/17 18:16, Ray Jui wrote:
>> Hi Marc,
>>
>> On 5/30/17 9:59 AM, Marc Zyngier wrote:
>>> On 30/05/17 17:49, Ray Jui wrote:
>>>> Hi Will,
>>>>
>>>> On 5/30/17 8:14 AM, Will Deacon wrote:
>>>>> On Mon, May 29, 2017 at 06:18:45PM -0700, Ray Jui wrote:
>>>>>> I'm writing to check with you to see if the latest arm-smmu.c driver in
>>>>>> v4.12-rc Linux for smmu-500 can support mapping that is only specific to
>>>>>> a particular physical address range while leave the rest still to be
>>>>>> handled by the client device. I believe this can already be supported by
>>>>>> the device tree binding of the generic IOMMU framework; however, it is
>>>>>> not clear to me whether or not the arm-smmu.c driver can support it.
>>>>>>
>>>>>> To give you some background information:
>>>>>>
>>>>>> We have a SoC that has PCIe root complex that has a build-in logic block
>>>>>> to forward MSI writes to ARM GICv3 ITS. Unfortunately, this logic block
>>>>>> has a HW bug that causes the MSI writes not parsed properly and can
>>>>>> potentially corrupt data in the internal FIFO. A workaround is to have
>>>>>> ARM MMU-500 takes care of all inbound transactions. I found that is
>>>>>> working after hooking up our PCIe root complex to MMU-500; however, even
>>>>>> with this optimized arm-smmu driver in v4.12, I'm still seeing a
>>>>>> significant Ethernet throughput drop in both the TX and RX directions.
>>>>>> The throughput drop is very significant at around 50% (but is already
>>>>>> much improved compared to other prior kernel versions at 70~90%).
>>>>>
>>>>> Did Robin's experiments help at all with this?
>>>>>
>>>>> http://www.linux-arm.org/git?p=linux-rm.git;a=shortlog;h=refs/heads/iommu/perf
>>>>>
>>>>
>>>> It looks like these are new optimizations that have not yet been merged
>>>> in v4.12? I'm going to give it a try.
>>>>
>>>>>> One alternative is to only use MMU-500 for MSI writes towards
>>>>>> GITS_TRANSLATER register in the GICv3, i.e., if I can define a specific
>>>>>> region of physical address that I want MMU-500 to act on and leave the
>>>>>> rest of inbound transactions to be handled directly by our PCIe
>>>>>> controller, it can potentially work around the HW bug we have and at the
>>>>>> same time achieve optimal throughput.
>>>>>
>>>>> I don't think you can bypass the SMMU for MSIs unless you give them their
>>>>> own StreamIDs, which is likely to break things horribly in the kernel. You
>>>>> could try to create an identity mapping, but you'll still have the
>>>>> translation overhead and you'd probably end up having to supply your own DMA
>>>>> ops to manage the address space. I'm assuming that you need to prevent the
>>>>> physical address of the ITS from being allocated as an IOVA?
>>>>
>>>> Will, is that a HW limitation that the SMMU cannot be used, only for MSI
>>>> writes, in which case, the physical address range is very specific in
>>>> our ASIC that falls in the device memory region (e.g., below 0x80000000)?
>>>>
>>>> In fact, what I need in this case is a static mapping from IOMMU on the
>>>> physical address of the GITS_TRANSLATER of the GICv3 ITS, which is the
>>>> address that MSI writes go to. This is to bypass the MSI forwarding
>>>> logic in our PCIe controller. At the same time, I can leave the rest of
>>>> inbound transactions to be handled by our PCIe controller without going
>>>> through the MMU.
>>>
>>> How is that going to work for DMA? I imagine your network interfaces do
>>> have to access memory, don't they? How can the transactions be
>>> terminated in the PCIe controller?
>>
>> Sorry, I may not phrase this properly. These inbound transactions (DMA
>> write to DDR, from endpoint) do not terminate in the PCIe controller.
>> They are taken by the PCIe controller as PCIe transactions and will be
>> carried towards the designated memory on the host.
> 
> So what is the StreamID used for these transactions? Is that a different
> StreamID from that of the DMAing device? If you want to avoid the SMMU
> effect on the transaction, you must make sure if doesn't match anything
> there.
> 
> Thanks,
> 
> 	M.
> 

Thanks for the reply. I'm checking with our ASIC team, but from my
understanding, the stream ID in our ASIC is constructed based on the
some custom fields that a developer can program + some standard PCIe BDF
fields. That is, I don't think we can make the stream ID from the same
PF different between MSI writes and DMA writes, as you have already
predicted.

It sounds like I do not have much option here...

Thanks,

Ray