Re: [PATCH] vfio: do not coalesce DMA mappings

["Gupta, Nipun" <nipun.gupta@amd.com>]

On 7/4/2023 7:39 PM, Thomas Monjalon wrote:
> 04/07/2023 11:23, Gupta, Nipun:
>> On 7/4/2023 1:36 PM, Ding, Xuan wrote:
>>> From: Gupta, Nipun <Nipun.Gupta@amd.com>
>>>> From: Ding, Xuan <xuan.ding@intel.com>
>>>>> From: Ding, Xuan
>>>>>> From: Nipun Gupta <nipun.gupta@amd.com>
>>>>>>> Hi Xuan,
>>>>>>>
>>>>>>> Thanks for pointing out the issue and figuring out the patch which
>>>>>>> introduced this. If you have answers to below queries, please let me know:
>>>>>>>
>>>>>>> Is there any other test cases which tests "--no-huge" which pass?
>>>>>>
>>>>>> Yes, there are test cases adding "--no-huge" option to validate 4k
>>>>>> page size in async vhost.
>>>>>> Actually, the page size is decided by front-end, so I think this
>>>>>> case can be removed.
>>>>>>
>>>>>> Previously, testpmd can start with "--no-huge" options (not sure if
>>>>>> there are test cases).
>>>>>> Cmd: ./build/app/dpdk-testpmd -l 5-6 -n 4 --no-huge -m 1024 -- -i
>>>>>>
>>>>>>>
>>>>>>> Also, if we change the "-m" option to provide lower memory, does
>>>>>>> the test pass?
>>>>>>
>>>>>> "-m" option is also added and does not work.
>>>>>>
>>>>>>>
>>>>>>> When you mention too many pages exceed the capability of IOMMU,
>>>>>>> you are referring to HW capability to create multiple pages? Here
>>>>>>> it seems in case of 4K page size we need 256K pages which is limiting the
>>>> capacity?
>>>>>>
>>>>>> Yes, this is the result of my initial debugging.
>>>>>> The direct impact is that this kind of testpmd cases cannot start now.
>>>>>> If this is expected, I think we can close this defect and ignore the "--no-
>>>> huge"
>>>>>> option when start.
>>>>>
>>>>> Any insights? Should we just ignore the "--no-huge" option and close this
>>>> defect?
>>>>> Now we did this as a workaround. Seems no one uses the "--no-huge"
>>>>> option in testpmd now.
>>>>
>>>> VFIO supports dma_entry_limit as a module parameter, which has a default
>>>> value of U16_MAX i.e. 64K, most likely which is limiting creation of 256K
>>>> entries for 4K pages here. This can be modified while inserting vfio module:
>>>>           modprobe vfio_iommu_type1 dma_entry_limit=1000000
>>>
>>> Thanks for your suggestion. I tried it on ubuntu 22.04 but it does not work.
>>> The reason I think is vfio-pci is build-in in kernel driver (since 20.04) and it does not support dynamic insmod/rmmod.
>>>
>>> Does this command need to rmmod vfio first and then modprobe again?
>>>
>>
>> If it is inserted as a module then you can remove using rmmod and then
>> modprobe again with the dma_entry_limit parameter. Also note,
>> vfio_iommu_type1 is the module which is limiting the entries to 64K, so
>> this module needs to be inserted again providing the dma_entry_limit
>> module param.
>>
>> In case the module is built-in you can provide via kernel command line
>> parameter (ref:
>> https://www.kernel.org/doc/html/v4.12/admin-guide/kernel-parameters.html).
>> As per this ref document, "vfio_iommu_type1.dma_entry_limit=1000000"
>> should be used in the bootargs to set the module parameters.
>>
>> FYI.. DPDK documentation also mentions the limitation at:
>> https://doc.dpdk.org/guides/linux_gsg/linux_drivers.html
> 
> Yes the parameter is discussed in
> https://doc.dpdk.org/guides/linux_gsg/linux_drivers.html#vfio-memory-mapping-limits
> but it does not mention we may need to decrease it with --no-huge.
> Please could you add this to the documentation?

sure! Ill send out a patch for this.

> 
>