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

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Date:   Fri, 5 Jul 2019 02:24:37 +0900
From:   Suwan Kim <suwan.kim027@...il.com>
To:     Alan Stern <stern@...land.harvard.edu>
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, 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.

Regards

Suwan Kim