Re: in-kernel verification of user PI?

[Christoph Hellwig <hch@lst.de>]

On Wed, Jan 29, 2025 at 09:23:37AM -0500, Martin K. Petersen wrote:
> Doing a verification pass in the write hot path had a substantial
> performance impact when I originally did this.

Oh yes, it absolutely will.  While the CRC implementations got a lot
faster in the last years, there's still a cost.  It also touches a lot of
cache lines.

> Even remapping the ref
> tag has an impact on cache. That's why DIX1.1 moved ref tag remapping to
> the HBA so we could avoid touching the PI buffer altogether in the hot
> path.

As in supplying an offset for the ref tag somewhere in the HBA specific
per-command payload?  That's not implemented in Linux as far as I can
tell, or did I miss something?

> > I.e. if userspace passes incorrect information it can trigger a
> > command failure and thus the driver error handler, which is something
> > we don't usually allow for "regular" I/O.
> 
> Do you trigger EH in NVMe? For SCSI we just bubble the PI error up
> without retrying.

We don't have the EH thread from hell, but there is error handling yes.