[RFC PATCH 0/3] Add an option for devices to skip SWIOTLB pre-copy on read.

[Robin Murphy]

On 2025-02-04 11:36 pm, Jianxiong Gao wrote:
> Removes an extra memory copy that occurs during IO read
> operations through the SWIOTLB. During high throughput
> read workloads, this extra copy is causing a lot of stress
> on the SWIOTLB.
> 
> With high performance devices, for example NVMe devices,
> the device will be overwriting the entire buffer. In such
> cases the entire pre-copy is redundent, only to slow down
> the overall bounce buffering.
> 
> We propose to add a full_buffer_write flag to the
> device_dma_parameters flag. When the flag is set the pre-copy
> can be omitted to boost performance.

No. We already went through this a couple of weeks ago[1]. It's not 
about what the driver intends the device to do, it's about what happens 
if for whatever reason it then *doesn't* do that.

Thanks,
Robin.

[1] 
https://lore.kernel.org/lkml/9582878b-1ce7-4fc4-9b45-b72bba722f49@arm.com/

> 
> Jianxiong Gao (3):
>    add full_buffer_write flag to struct device_dma_parameters
>    skip swiotlb pre copy if the device does full buffer write.
>    set full_buffer_write for nvme devices.
> 
>   drivers/nvme/host/pci.c     |  1 +
>   include/linux/device.h      |  1 +
>   include/linux/dma-mapping.h | 15 +++++++++++++++
>   kernel/dma/swiotlb.c        |  3 ++-
>   4 files changed, 19 insertions(+), 1 deletion(-)
>