Re: [PATCH v15 00/12] SMMUv3 Nested Stage Setup (IOMMU part)

[Xingang Wang]

Hi Eric, Jean-Philippe

On 2021/4/11 19:12, Eric Auger wrote:
> SMMUv3 Nested Stage Setup (IOMMU part)
>
> This series brings the IOMMU part of HW nested paging support
> in the SMMUv3. The VFIO part is submitted separately.
>
> This is based on Jean-Philippe's
> [PATCH v14 00/10] iommu: I/O page faults for SMMUv3
> https://www.spinics.net/lists/arm-kernel/msg886518.html
> (including the patches that were not pulled for 5.13)
>
> The IOMMU API is extended to support 2 new API functionalities:
> 1) pass the guest stage 1 configuration
> 2) pass stage 1 MSI bindings
>
> Then those capabilities gets implemented in the SMMUv3 driver.
>
> The virtualizer passes information through the VFIO user API
> which cascades them to the iommu subsystem. This allows the guest
> to own stage 1 tables and context descriptors (so-called PASID
> table) while the host owns stage 2 tables and main configuration
> structures (STE).
>
> Best Regards
>
> Eric
>
> This series can be found at:
> v5.12-rc6-jean-iopf-14-2stage-v15
> (including the VFIO part in its last version: v13)

I am testing the performance of an accelerator with/without SVA/vSVA,
and found there might be some potential performance loss risk for SVA/vSVA.

I use a Network and computing encryption device (SEC), and send 1MB 
request for 10000 times.

I trigger mm fault before I send the request, so there should be no iopf.

Here's what I got:

physical scenario:
performance:		SVA:9MB/s  	NOSVA:9MB/s
tlb_miss: 		SVA:302,651	NOSVA:1,223
trans_table_walk_access:SVA:302,276	NOSVA:1,237

VM scenario:
performance:		vSVA:9MB/s  	NOvSVA:6MB/s  about 30~40% loss
tlb_miss: 		vSVA:4,423,897	NOvSVA:1,907
trans_table_walk_access:vSVA:61,928,430	NOvSVA:21,948

In physical scenario, there's almost no performance loss, but the 
tlb_miss and trans_table_walk_access of stage 1 for SVA is quite high, 
comparing to NOSVA.

In VM scenario, there's about 30~40% performance loss, this is because 
the two stage tlb_miss and trans_table_walk_access is even higher, and 
impact the performance.

I compare the procedure of building page table of SVA and NOSVA, and 
found that NOSVA uses 2MB mapping as far as possible, while SVA uses 
only 4KB.

I retest with huge page, and huge page could solve this problem, the 
performance of SVA/vSVA is almost the same as NOSVA.

I am wondering do you have any other solution for the performance loss 
of vSVA, or any other method to reduce the tlb_miss/trans_table_walk.

Thanks

Xingang

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