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
