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Date:   Fri, 5 Jul 2019 18:07:09 +0900
From:   Suwan Kim <suwan.kim027@gmail.com>
To:     Alan Stern <stern@rowland.harvard.edu>
Cc:     shuah@kernel.org, gregkh@linuxfoundation.org,
        linux-usb@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 2/2] usbip: Implement SG support to vhci
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On Thu, Jul 04, 2019 at 09:41:04PM -0400, Alan Stern wrote:
> On Fri, 5 Jul 2019, Suwan Kim wrote:
> 
> > On Mon, Jun 24, 2019 at 01:24:15PM -0400, Alan Stern wrote:
> > > On Mon, 24 Jun 2019, Suwan Kim wrote:
> > > 
> > > > > > +	hcd->self.sg_tablesize = ~0;
> > > > > > +	hcd->self.no_sg_constraint = 1;
> > > > > 
> > > > > You probably shouldn't do this, for two reasons.  First, sg_tablesize
> > > > > of the server's HCD may be smaller than ~0.  If the client's value is
> > > > > larger than the server's, a transfer could be accepted on the client
> > > > > but then fail on the server because the SG list was too big.
> > > 
> > > On the other hand, I don't know of any examples where an HCD has 
> > > sg_tablesize set to anything other than 0 or ~0.  vhci-hcd might end up 
> > > being the only one.
> > > 
> > > > > Also, you may want to restrict the size of SG transfers even further,
> > > > > so that you don't have to allocate a tremendous amount of memory all at
> > > > > once on the server.  An SG transfer can be quite large.  I don't know 
> > > > > what a reasonable limit would be -- 16 perhaps?
> > > > 
> > > > Is there any reason why you think that 16 is ok? Or Can I set this
> > > > value as the smallest value of all HC? I think that sg_tablesize
> > > > cannot be a variable value because vhci interacts with different
> > > > machines and all machines has different sg_tablesize value.
> > > 
> > > I didn't have any good reason for picking 16.  Using the smallest value 
> > > of all the HCDs seems like a good idea.
> > 
> > I also have not seen an HCD with a value other than ~0 or 0 except for
> > whci which uses 2048, but is not 2048 the maximum value of sg_tablesize?
> > If so, ~0 is the minimum value of sg_tablesize that supports SG. Then
> > can vhci use ~0 if we don't consider memory pressure of the server?
> > 
> > If all of the HCDs supporting SG have ~0 as sg_tablesize value, I
> > think that whether we use an HCD locally or remotely, the degree of
> > memory pressure is same in both local and remote usage.
> 
> You have a lot of leeway.  For example, there's no reason a single SG
> transfer on the client has to correspond to a single SG transfer on the
> host.  In theory the client's vhci-hcd can break a large SG transfer up
> into a bunch of smaller pieces and send them to the host one by one,
> then reassemble the results to complete the original transfer.  That
> way the memory pressure on the host would be a lot smaller than on the
> client.

Thank you for the feedback, Alan. I understood your comment. It
seems to be a good idea to use sg_tablesize to alleviate the memory
pressure of the host. But I think 16 is too small for USB 3.0 device
because USB 3.0 storage device in my machine usually uses more than
30 SG entries. So, I will set sg_tablesize to 32.

Regards

Suwan Kim