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

From: Alan Stern
Date: Mon Jun 24 2019 - 13:24:17 EST

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