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.leiz...@huawei.com/
Signed-off-by: John Garry <john.ga...@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.
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
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