Re: [PATCH v12 0/6] iommu/dma: s390 DMA API conversion and optimized IOTLB flushing

Matthew Rosato
2023-09-27T20:26:43Z

On 9/27/23 11:40 AM, Jason Gunthorpe wrote:
> On Wed, Sep 27, 2023 at 05:24:20PM +0200, Niklas Schnelle wrote:
>
>> Ok, another update. On trying it out again this problem actually also
>> occurs when applying this v12 on top of v6.6-rc3 too. Also I guess
>> unlike my prior thinking it probably doesn't occur with
>> iommu.forcedac=1 since that still allows IOVAs below 4 GiB and we might
>> be the only ones who don't support those. From my point of view this
>> sounds like a mlx5_core issue they really should call
>> dma_set_mask_and_coherent() before their first call to
>> dma_alloc_coherent() not after. So I guess I'll send a v13 of this
>> series rebased on iommu/core and with an additional mlx5 patch and then
>> let's hope we can get that merged in a way that doesn't leave us with
>> broken ConnectX VFs for too long.
>
> Yes, OK. It definitely sounds wrong that mlx5 is doing dma allocations before
> setting it's dma_set_mask_and_coherent(). Please link to this thread
> and we can get Leon or Saeed to ack it for Joerg.
>

Hi Niklas,

I bisected the start of this issue to the following commit (only noticeable on s390 when you apply this subject series on top):

06cd555f73caec515a14d42ef052221fa2587ff9 ("net/mlx5: split mlx5_cmd_init() to probe and reload routines")

Which went in during the merge window. Please include with your fix and/or report to the mlx5 maintainers. Looks like the changes in this patch match what you and Jason describe; it splits up mlx5_cmd_init() and moves part of the call earlier. The net result is we first call mlx5_mdev_init>mlx5_cmd_init->alloc_cmd_page->dma_alloc_coherent and then sometime later call mlx5_pci_init->set_dma_caps->dma_set_mask_and_coherent.

Prior to this patch, we would not drive mlx5_cmd_init (and thus that first dma_alloc_coherent) until mlx5_init_one which happens _after_ mlx5_pci_init->set_dma_caps->dma_set_mask_and_coherent.

Thanks,
Matt