linux-kernel - Re: [PATCH 2/2] usbip: Implement SG support to vhci

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Date:   Mon, 24 Jun 2019 13:24:15 -0400 (EDT)
From:   Alan Stern <stern@...land.harvard.edu>
To:     Suwan Kim <suwan.kim027@...il.com>
cc:     shuah@...nel.org, <gregkh@...uxfoundation.org>,
        <linux-usb@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] usbip: Implement SG support to vhci

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.

Alan Stern