Re: [PATCH v8] usbip: Implement SG support to vhci-hcd and stub driver

From: shuah
Date: Tue Aug 27 2019 - 10:38:52 EST

On 8/26/19 11:23 AM, Suwan Kim wrote:
There are bugs on vhci with usb 3.0 storage device. In USB, each SG
list entry buffer should be divisible by the bulk max packet size.
But with native SG support, this problem doesn't matter because the
SG buffer is treated as contiguous buffer. But without native SG
support, USB storage driver breaks SG list into several URBs and the
error occurs because of a buffer size of URB that cannot be divided
by the bulk max packet size. The error situation is as follows.

When USB Storage driver requests 31.5 KB data and has SG list which
has 3584 bytes buffer followed by 7 4096 bytes buffer for some
reason. USB Storage driver splits this SG list into several URBs
because VHCI doesn't support SG and sends them separately. So the
first URB buffer size is 3584 bytes. When receiving data from device,
USB 3.0 device sends data packet of 1024 bytes size because the max
packet size of BULK pipe is 1024 bytes. So device sends 4096 bytes.
But the first URB buffer has only 3584 bytes buffer size. So host
controller terminates the transfer even though there is more data to
receive. So, vhci needs to support SG transfer to prevent this error.

In this patch, vhci supports SG regardless of whether the server's
host controller supports SG or not, because stub driver splits SG
list into several URBs if the server's host controller doesn't
support SG.

To support SG, vhci sets URB_DMA_MAP_SG flag in transfer_flags of
usbip header if URB has SG list and this flag will tell stub driver
to use SG list.

vhci sends each SG list entry to stub driver. Then, stub driver sees
the total length of the buffer and allocates SG table and pages
according to the total buffer length calling sgl_alloc(). After stub
driver receives completed URB, it again sends each SG list entry to

If the server's host controller doesn't support SG, stub driver
breaks a single SG request into several URBs and submits them to
the server's host controller. When all the split URBs are completed,
stub driver reassembles the URBs into a single return command and
sends it to vhci.

Moreover, in the situation where vhci supports SG, but stub driver
does not, or vice versa, usbip works normally. Because there is no
protocol modification, there is no problem in communication between
server and client even if the one has a kernel without SG support.

In the case of vhci supports SG and stub driver doesn't, because
vhci sends only the total length of the buffer to stub driver as
it did before the patch applied, stub driver only needs to allocate
the required length of buffers using only kmalloc() regardless of
whether vhci supports SG or not. But stub driver has to allocate
buffer with kmalloc() as much as the total length of SG buffer which
is quite huge when vhci sends SG request, so it has overhead in
buffer allocation in this situation.

If stub driver needs to send data buffer to vhci because of IN pipe,
stub driver also sends only total length of buffer as metadata and
then sends real data as vhci does. Then vhci receive data from stub
driver and store it to the corresponding buffer of SG list entry.

And for the case of stub driver supports SG and vhci doesn't, since
the USB storage driver checks that vhci doesn't support SG and sends
the request to stub driver by splitting the SG list into multiple
URBs, stub driver allocates a buffer for each URB with kmalloc() as
it did before this patch.

* Test environment

Test uses two difference machines and two different kernel version
to make mismatch situation between the client and the server where
vhci supports SG, but stub driver does not, or vice versa. All tests
are conducted in both full SG support that both vhci and stub support
SG and half SG support that is the mismatch situation. Test kernel
version is 5.3-rc6 with commit "usb: add a HCD_DMA flag instead of
guestimating DMA capabilities" to avoid unnecessary DMA mapping and

- Test kernel version
- 5.3-rc6 with SG support
- 5.1.20-200.fc29.x86_64 without SG support

* SG support test

- Test devices
- Super-speed storage device - SanDisk Ultra USB 3.0
- High-speed storage device - SMI corporation USB 2.0 flash drive

- Test description

Test read and write operation of mass storage device that uses the
BULK transfer. In test, the client reads and writes files whose size
is over 1G and it works normally.

* Regression test

- Test devices
- Super-speed device - Logitech Brio webcam
- High-speed device - Logitech C920 HD Pro webcam
- Full-speed device - Logitech bluetooth mouse
- Britz BR-Orion speaker
- Low-speed device - Logitech wired mouse

- Test description

Moving and click test for mouse. To test the webcam, use gnome-cheese.
To test the speaker, play music and video on the client. All works

* VUDC compatibility test

VUDC also works well with this patch. Tests are done with two USB
gadget created by CONFIGFS USB gadget. Both use the BULK pipe.

1. Serial gadget
2. Mass storage gadget

- Serial gadget test

Serial gadget on the host sends and receives data using cat command
on the /dev/ttyGS<N>. The client uses minicom to communicate with
the serial gadget.

- Mass storage gadget test

After connecting the gadget with vhci, use "dd" to test read and
write operation on the client side.

Read - dd if=/dev/sd<N> iflag=direct of=/dev/null bs=1G count=1
Write - dd if=<my file path> iflag=direct of=/dev/sd<N> bs=1G count=1

Signed-off-by: Suwan Kim <suwan.kim027@xxxxxxxxx>
v7 - v8
- Modify the commit log which describes URB_DMA_MAP_SG flag setting.

v6 - v7
- Move the flag set in setup_cmd_submit_pdu() of vhci_tx.c and
manipulate usbip header flag instead of urb->transfer_flags.

- Remove clearing URB_DMA_MAP_SG flag in vhci_rx.

setup_cmd_submit_pdu() is just for pdu and shouldn't be concerned
about the urb.

Please keep the URB_DMA_MAP_SG setting in urb->transfer_flags.
That mean you are restoring v6 code change with the commit log
updates from v8.

This flag belongs with urb and not the cmd.submit. Having it in
urb also helps when we debug and dump the urb from usbip_dump_urb()

-- Shuah