Re: How to resolve an issue in swiotlb environment?

Suwan Kim
2019-06-21T18:49:29Z

On Wed, Jun 19, 2019 at 05:05:49PM -0400, Alan Stern wrote:
> On Wed, 19 Jun 2019, shuah wrote:
>
> > I missed a lot of the thread info. and went looking for it and found the
> > following summary of the problem:
> >
> > ==================
> > The issue which prompted the commit this thread is about arose in a
> > situation where the block layer set up a scatterlist containing buffer
> > sizes something like:
> >
> > 4096 4096 1536 1024
> >
> > and the maximum packet size was 1024. The situation was a little
> > unusual, because it involved vhci-hcd (a virtual HCD). This doesn't
> > matter much in normal practice because:
> >
> > Block devices normally have a block size of 512 bytes or more.
> > Smaller values are very uncommon. So scatterlist element sizes
> > are always divisible by 512.
> >
> > xHCI is the only USB host controller type with a maximum packet
> > size larger than 512, and xHCI hardware can do full
> > scatter-gather so it doesn't care what the buffer sizes are.
> >
> > So another approach would be to fix vhci-hcd and then trust that the
> > problem won't arise again, for the reasons above. We would be okay so
> > long as nobody tried to use a USB-SCSI device with a block size of 256
> > bytes or less.
> > ===================
> >
> > Out of the summary, the following gives me pause:
> >
> > "xHCI hardware can do full scatter-gather so it doesn't care what the
> > buffer sizes are."
> >
> > vhci-hcd won't be able to count on hardware being able to do full
> > scatter-gather. It has to deal with a variety of hardware with
> > varying speeds.
>
> Sure. But you can test whether the server's HCD is able to handle
> scatter-gather transfers, and if it is then you can say that the
> client-side vhci-hcd is able to handle them as well. Then all you
> would have to do is preserve the scatterlist information describing the
> transfer when you go between the client and the server.
>
> The point is to make sure that the client-side vhci-hcd doesn't claim
> to be _less_ capable than the server-side actual HCD. That's what
> leads to the problem described above.
>
> > "We would be okay so long as nobody tried to use a USB-SCSI device with
> > a block size of 256 bytes or less."
> >
> > At least a USB Storage device, I test with says 512 block size. Can we
> > count on not seeing a device with block size <= 256 bytes?
>
> Yes, we can. In fact, the SCSI core doesn't handle devices with block
> size < 512.
>
> > In any case, I am looking into adding SG support vhci-hci at the moment.
> >
> > Looks like the following is the repo, I should be working with?
> >
> > git://git.infradead.org/users/hch/misc.git
>
> It doesn't matter. Your work should end up being independent of
> Christoph's, so you can base it on any repo.

I implemented SG support of vhci. I will send it as a patch.
Please look at it and let me know if you have a feedback.

Regards

Suwan Kim