[Impact]
The Intel IOMMU driver provides an option for strict mode. When disabled, batching of IOTLB flush operations is permitted, allowing the user to trade-off isolation for improved performance. Ubuntu's kernel currently lacks a parity for this feature for ARM.
There's a significant performance gain to be had by removing the need to flush the IOMMU TLB on every unmap on arm64. I'm seeing a 25% performance gain w/ fio reads on a single NVMe device.
This mode of operation is available for x86 via the "intel_iommu=strict" parameter. Upstream now exposes an equivalent feature for ARM platforms via the "iommu.strict=[0|1]" parameter, while retaining the default strict-enabled mode.
[Test Case]
Run fio with the following config before and after applying the patches and collection IOPS count. Run again after applying the patches. Finally, run a 3rd time after adding iommu.strict=0 to the kernel commandline.
Performance should not regress after the update. Performance should further improve after adding iommu.strict=0 - but if it doesn't for some reason, that is not a regression.
$ cat fio.rc
[global]
rw=read
direct=1
ioengine=libaio
iodepth=2048
numjobs=10
bs=4k
group_reporting=1
group_reporting=1
cpumask=0xff
runtime=100
loops = 10000
[job1]
filename=/dev/nvme0n1
[Fix]
44f6876a00e83 iommu/arm-smmu: Support non-strict mode
b2dfeba654cb0 iommu/io-pgtable-arm-v7s: Add support for non-strict mode
9662b99a19abc iommu/arm-smmu-v3: Add support for non-strict mode
b6b65ca20bc93 iommu/io-pgtable-arm: Add support for non-strict mode
68a6efe86f6a1 iommu: Add "iommu.strict" command line option
2da274cdf998a iommu/dma: Add support for non-strict mode
07fdef34d2be6 iommu/arm-smmu-v3: Implement flush_iotlb_all hook
85c7a0f1ef624 iommu/io-pgtable-arm: Fix race handling in split_blk_unmap()
[Regression Risk]
Most of these patches are specific to ARM, and have been regression tested on both arm64 (HiSilicon D06) and armhf (QEMU virt) using "stress-ng --vm $(nproc)"
2 patches do touch arch-indep code however:
> 68a6efe86f6a1 iommu: Add "iommu.strict" command line option
Adds a new command line option and sets an attribute that iommu drivers can optionally react to. Doesn't change default behavior.
> 2da274cdf998a iommu/dma: Add support for non-strict mode
This driver is only built for arm64 and ppc64el (determined by looking at the build logs). Most of this patch only changes behavior in the non-default (and new) iommu.strict=0 case. The exception, which is called out in the commit message, is this hunk:
- WARN_ON(iommu_unmap(domain, dma_addr, size) != size);
+ WARN_ON(iommu_unmap_fast(domain, dma_addr, size) != size);
+ if (!cookie->fq_domain)
+ iommu_tlb_sync(domain);
In the default case, where fq_domain will be NULl, we are now factoring iommu_unmap() into:
iommu_unmap_fast()
iommu_tlb_sync()
Looking at the source to iommu_unmap() confirms that this is functionally equivalent.
This bug is awaiting verification that the kernel in -proposed solves the problem. Please test the kernel and update this bug with the results. If the problem is solved, change the tag 'verification-
If verification is not done by 5 working days from today, this fix will be dropped from the source code, and this bug will be closed.
See https:/