On 2021-06-02 09:09, Daniel Borkmann wrote:

On 6/1/21 7:42 PM, Jussi Maki wrote:

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Hi Robin,

On Tue, Jun 1, 2021 at 2:39 PM Robin Murphy [off-list ref] wrote:

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The regression shows as a significant drop in throughput as measured
with "super_netperf" [0],
with measured bandwidth of ~95Gbps before and ~35Gbps after:

I guess that must be the difference between using the flush queue
vs. strict invalidation. On closer inspection, it seems to me that
there's a subtle pre-existing bug in the AMD IOMMU driver, in that
amd_iommu_init_dma_ops() actually runs *after* amd_iommu_init_api()
has called bus_set_iommu(). Does the patch below work?

Thanks for the quick response & patch. I tried it out and indeed it
does solve the issue:

Cool, thanks Jussi. May I infer a Tested-by tag from that?

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# uname -a
Linux zh-lab-node-3 5.13.0-rc3-amd-iommu+ #31 SMP Tue Jun 1 17:12:57
UTC 2021 x86_64 x86_64 x86_64 GNU/Linux
root@zh-lab-node-3:~# ./super_netperf 32 -H 172.18.0.2
95341.2

root@zh-lab-node-3:~# uname -a
Linux zh-lab-node-3 5.13.0-rc3-amd-iommu-unpatched #32 SMP Tue Jun 1
17:29:34 UTC 2021 x86_64 x86_64 x86_64 GNU/Linux
root@zh-lab-node-3:~# ./super_netperf 32 -H 172.18.0.2
33989.5

Robin, probably goes without saying, but please make sure to include ...

Fixes: a250c23f15c2 ("iommu: remove DOMAIN_ATTR_DMA_USE_FLUSH_QUEUE")

... to your fix in [0], maybe along with another Fixes tag pointing to 
the original
commit adding this issue. But certainly a250c23f15c2 would be good given 
the regression
was uncovered on that one first, so that Greg et al have a chance to 
pick this fix up
for stable kernels.

Given that the race looks to have been pretty theoretical until now, I'm 
not convinced it's worth the bother of digging through the long history 
of default domain and DMA ops movement to figure where it started, much 
less attempt invasive backports. The flush queue change which made it 
apparent only landed in 5.13-rc1, so as long as we can get this in as a 
fix in the current cycle we should be golden - in the meantime, note 
that booting with "iommu.strict=0" should also restore the expected 
behaviour.

FWIW I do still plan to resend the patch "properly" soon (in all honesty 
it wasn't even compile-tested!)

Cheers,
Robin.