Re: [RFC PATCH] dma-iommu: Add iommu_dma_max_mapping_size()

[John Garry]

On 17/05/2022 11:40, Robin Murphy wrote:
> On 2022-05-16 14:06, John Garry wrote:
> > For streaming DMA mappings involving an IOMMU and whose IOVA len 
> > regularly
> > exceeds the IOVA rcache upper limit (meaning that they are not cached),
> > performance can be reduced.
> >
> > Add the IOMMU callback for DMA mapping API dma_max_mapping_size(), which
> > allows the drivers to know the mapping limit and thus limit the requested
> > IOVA lengths.
> >
> > This resolves the performance issue originally reported in [0] for a SCSI
> > HBA driver which was regularly mapping SGLs which required IOVAs in
> > excess of the IOVA caching limit. In this case the block layer limits the
> > max sectors per request - as configured in __scsi_init_queue() - which
> > will limit the total SGL length the driver tries to map and in turn 
> > limits
> > IOVA lengths requested.
> >
> > [0] 
> > https://lore.kernel.org/linux-iommu/20210129092120.1482-1-thunder.leizhen@xxxxxxxxxx/ 
> >
> >
> > Signed-off-by: John Garry <john.garry@xxxxxxxxxx>
> > ---
> > Sending as an RFC as iommu_dma_max_mapping_size() is a soft limit, and 
> > not
> > a hard limit which I expect is the semantics of 
> > dma_map_ops.max_mapping_size
>
> Indeed, sorry but NAK for this being nonsense. As I've said at least 
> once before, if the unnecessary SAC address allocation attempt slows 
> down your workload, make it not do that in the first place. If you don't 
> like the existing command-line parameter then fine, > there are plenty of
> other options, it just needs to be done in a way that doesn't break x86 
> systems with dodgy firmware, as my first attempt turned out to.

Sorry, but I am not interested in this. It was discussed in Jan last 
year without any viable solution.

Anyway we still have the long-term IOVA aging issue, and requesting 
non-cached IOVAs is involved in that. So I would rather keep the SCSI 
driver to requesting cached IOVAs all the time.

I did try to do it the other way around - configuring the IOVA caching 
range according to the drivers requirement but that got nowhere.

> Furthermore, if a particular SCSI driver doesn't benefit from mappings 
> larger than 256KB, then that driver is also free to limit its own 
> mapping size. There are other folks out there with use-cases for mapping 
> *gigabytes* at once; you don't get to cripple the API and say that 
> that's suddenly not allowed just because it happens to make your thing 
> go faster, that's absurd.

I'd say less catastrophically slow, not faster.

So how to inform the SCSI driver of this caching limit then so that it 
may limit the SGL length?

Thanks,
John