[PATCH 0/2] Unprivileged sgl-only passthrough

[Kanchan Joshi]

On Thu, Oct 19, 2023 at 12:10 AM Jens Axboe <axboe at kernel.dk> wrote:
>
> On 10/18/23 12:30 PM, Kanchan Joshi wrote:
> > Patch 1: Prep. Adds the meta-transfer ability in nvme-pci
> > Patch 2: Enables fine-granular passthrough with the change that i/o
> > commands can transfer the data only via SGL.
> >
> > Requirement:
> > - Prepared against block 6.6 tree.
> > - The patch in uring-passthrough failure handling is required to see the
> >   submission failure (if any)
> >   https://lore.kernel.org/linux-nvme/20231018135718.28820-1-joshi.k@samsung.com/
>
> I didn't have time to follow the previous discussion, but what's the
> reasoning behind allowing it for SGL only?

This was a solution that emerged while discussing how best to fill the
DMA corruption hole for passthrough.
With SGL, the buffer length (data/buffer) sanity checks are done by
the SSD and it fails the IO rather than doing extra transfer.

> IIRC, we do have an inline
> vec for a small number of vecs, so presumably this would not hit
> alloc+free for each IO?

16b dma_pool_alloc/free for each IO that involves metadata. This is to
keep the nvme-sgl that points to the metadata buffer.
Hopefully some ideas can emerge (during the review) to see if we can
do away with it.

>But even so, I would imagine that SGL is slower
> than PRP? Do we know how much?

I do not know at the moment. Plan is to evaluate this soon.

BTW, SGL-only mode is for unprivileged users only. For root, it
remains the same as before (prp or sgl depending on the data-transfer
length).