Re: How to resolve an issue in swiotlb environment?

From: shuah <shuah@kernel.org>
To: Alan Stern <stern@rowland.harvard.edu>
Cc: Linux-Renesas <linux-renesas-soc@vger.kernel.org>,
	"linux-usb@vger.kernel.org" <linux-usb@vger.kernel.org>,
	Oliver Neukum <oneukum@suse.com>,
	Valentina Manea <valentina.manea.m@gmail.com>,
	"linux-block@vger.kernel.org" <linux-block@vger.kernel.org>,
	"iommu@lists.linux-foundation.org"
	<iommu@lists.linux-foundation.org>, shuah <shuah@kernel.org>,
	Christoph Hellwig <hch@lst.de>
Subject: Re: How to resolve an issue in swiotlb environment?
Date: Wed, 19 Jun 2019 14:23:11 -0600	[thread overview]
Message-ID: <7a6450cd-7b68-778d-0124-3c21d4616069@kernel.org> (raw)
In-Reply-To: <20684ef6-7da7-90ed-4cd6-3bc46202de6d@kernel.org>

Hi Alan,

On 6/18/19 9:28 AM, shuah wrote:
> On 6/14/19 8:44 AM, Alan Stern wrote:
>> On Thu, 13 Jun 2019, shuah wrote:
>>
>>>> Great!  So all we have to do is fix vhci-hcd.  Then we can remove all
>>>> the virt_boundary_mask stuff from usb-storage and uas entirely.
>>>>
>>>> (I'm assuming wireless USB isn't a genuine issue.  As far as I know, it
>>>> is pretty much abandoned at this point.)
>>>>
>>>> Valentina and Shua: Adding SG support to vhci-hcd shouldn't be too
>>>> hard.  It ought to be possible even without changing the network
>>>> protocol.
>>>>
>>>
>>> I will start taking a look at this. Is there a target release in plan
>>> to drop virt_boundary_mask stuff?
>>
>> Not yet.  But since it doesn't do what we want anyway, this should be
>> fixed quickly.
>>
> 

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.

"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?

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

thanks,
-- Shuah
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