Re: [PATCH v4 2/2] usbip: Implement SG support to vhci-hcd and stub driver

[shuah]

On 8/6/19 9:48 AM, Suwan Kim wrote:
> On Tue, Aug 06, 2019 at 09:13:54AM -0600, shuah wrote:
> > On 8/6/19 6:31 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_map_urb_for_dma() sets URB_DMA_MAP_SG flag in
> > > urb->transfer_flags 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
> > > vhci.
> > >
> > > 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 regardless of whether vhci supports
> > > SG or not.
> > >
> > > 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.
> > >
> > > In the case of stub driver that supports SG, buffer is allocated by
> > > sgl_alloc(). However, stub driver that does not support SG allocates
> > > buffer using only kmalloc(). Therefore, if vhci supports SG and stub
> > > driver doesn't, 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 big overhead in buffer allocation.
> > >
> > > 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 with kmalloc() as it did before
> > > this patch.
> > >
> > > 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
> >
> > Thanks for the test results.
> >
> > Were you able to test with USB lowspeed devices?
>
> I tested USB3 super-speed device and USB2 high-speed device.
> But I don't have full/low speed device that uses BULK transfer.
> In USB spec, low-speed device doesn't support BULK transfer.

I think I mentioned earlier that the reason to test with lowspeed
is make sure there are no regressions due this change. You aren't
testing BULK transfer, you are looking for any regressions.

> Do I add test description about the USB3 super-speed and USB2
> high-speed device to the commit log?

Please do.

thanks,
-- Shuah