How to resolve an issue in swiotlb environment?

[Christoph Hellwig]

Hi Yoshihiro,

sorry for not taking care of this earlier, today is a public holiday
here and thus I'm not working much over the long weekend.

On Mon, Jun 10, 2019 at 11:13:07AM +0000, Yoshihiro Shimoda wrote:
> I have another way to avoid the issue. But it doesn't seem that a good way though...
> According to the commit that adding blk_queue_virt_boundary() [3],
> this is needed for vhci_hcd as a workaround so that if we avoid to call it
> on xhci-hcd driver, the issue disappeared. What do you think?
> JFYI, I pasted a tentative patch in the end of email [4].

Oh, I hadn't even look at why USB uses blk_queue_virt_boundary, and it
seems like the usage is wrong, as it doesn't follow the same rules as
all the others.  I think your patch goes in the right direction,
but instead of comparing a hcd name it needs to be keyed of a flag
set by the driver (I suspect there is one indicating native SG support,
but I can't quickly find it), and we need an alternative solution
for drivers that don't see like vhci.  I suspect just limiting the
entire transfer size to something that works for a single packet
for them would be fine.