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

[Robin Murphy]

On 27/09/2023 4:40 pm, 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.
> 
> (though wondering why s390 is the only case that ever hit this?)

Probably because most systems happen to be able to satisfy the 
allocation within the default 32-bit mask - the whole bottom 4GB of IOVA 
space being reserved is pretty atypical.

TBH it makes me wonder the opposite - how this ever worked on s390 
before? And I think the answer to that is "by pure chance", since upon 
inspection the existing s390_pci_dma_ops implementation appears to pay 
absolutely no attention to the device's DMA masks whatsoever :(

Robin.