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

[Thomas Monjalon <thomas@monjalon.net>]

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?