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

[Robin Murphy <robin.murphy@arm.com>]

On 2022-05-17 12:26, John Garry wrote:
> 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@huawei.com/ 
>>>
>>>
>>> Signed-off-by: John Garry <john.garry@huawei.com>
>>> ---
>>> 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.

Er, OK, if you're not interested in solving that problem I don't see why 
you'd bring it up, but hey ho. *I* still think it's important, so I 
guess I'll revive my old patch with a CONFIG_X86 bodge and have another 
go at some point.

> 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.

FWIW I thought that all looked OK, it just kept getting drowned out by 
more critical things in my inbox so I hoped someone else might comment. 
If it turns out that I've become the de-facto IOVA maintainer in 
everyone else's minds now and they're all waiting for my word then fair 
enough, I just need to know and reset my expectations accordingly. Joerg?

>> 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?

Driver-specific mechanism; block-layer-specific mechanism; redefine this 
whole API to something like dma_opt_mapping_size(), as a limit above 
which mappings might become less efficient or start to fail (callback to 
my thoughts on [1] as well, I suppose); many options. Just not imposing 
a ridiculously low *maximum* on everyone wherein mapping calls "should 
not be larger than the returned value" when that's clearly bollocks.

Cheers,
Robin.

[1] 
https://lore.kernel.org/linux-iommu/20220510142109.777738-1-ltykernel@gmail.com/
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