Re: [PATCH v2 00/11] Connect VFIO to IOMMUFD

[Matthew Rosato <mjrosato@linux.ibm.com>]

On 11/14/22 9:59 AM, Jason Gunthorpe wrote:
> On Mon, Nov 14, 2022 at 09:55:21AM -0500, Matthew Rosato wrote:
>>>> AFAICT there is no equivalent means to specify
>>>> vfio_iommu_type1.dma_entry_limit when using iommufd; looks like
>>>> we'll just always get the default 65535.
>>>
>>> No, there is no arbitary limit on iommufd
>>
>> Yeah, that's what I suspected.  But FWIW, userspace checks the
>> advertised limit via VFIO_IOMMU_GET_INFO /
>> VFIO_IOMMU_TYPE1_INFO_DMA_AVAIL, and this is still being advertised
>> as 65535 when using iommufd.  I don't think there is a defined way
>> to return 'ignore this value'.
> 
> Is something using this? Should we make it much bigger?

Yes, s390 when doing lazy unmapping likes to use larger amounts of concurrent DMA, so there can be cases where we want to raise this limit.

The initial value of 65535 is already pretty arbitrary (U16_MAX) -- If iommufd is doing its own management and this value becomes deprecated in this scenario, and we can't set it to a magic value that says 'ignore me' then maybe it just makes sense for now to set it arbitrarily larger when using iommufd e.g. U32_MAX?