On Fri, Feb 10, 2023 at 10:18:08AM -0800, Bart Van Assche wrote:
>On 2/10/23 10:00, Kanchan Joshi wrote:
>>3. DMA cost: is high in presence of IOMMU. Keith posted the work[1],
>>with block IO path, last year. I imagine plumbing to get a bit simpler
>>with passthrough-only support. But what are the other things that must
>>be sorted out to have progress on moving DMA cost out of the fast path?
>
>Are performance numbers available?

Around 55% decline when I checked last (6.1-rcX kernel).
512b randread IOPS with optane, on AMD ryzen 9 box -
when iommu is set to lazy (default config)= 3.1M
when iommmu is disabled or in passthrough mode = 4.9M

>Isn't IOMMU cost something that has already been solved? From https://www.usenix.org/system/files/conference/atc15/atc15-paper-peleg.pdf: 
>"Evaluation of our designs under Linux shows that (1)
>they achieve 88.5%–100% of the performance obtained
>without an IOMMU".

Since above numbers are more recent than the paper, this is yet to be
solved.