On Tue, May 19, 2020 at 07:33:04PM -0700, Alan Mikhak wrote: > Hi Lorenzo, > > I came across this issue when implementing a Linux NVMe endpoint function > driver under the Linux PCI Endpoint Framework: > https://lwn.net/Articles/804369/ > > I needed to map up to 128GB of host memory using a single ATU window > from the endpoint side because NVMe PRPs can be scattered all over host > memory. In the process, I came across this 4GB limitation where the > maximum size of memory that can be mapped is limited by what a u32 value > can represent. > > I submitted a separate patch to fix an undefined behavior that may also > happen in dw_pcie_prog_outbound_atu_unroll() under some circumstances > when the size of the memory being mapped is greater than what a u32 value > can represent. > https://patchwork.kernel.org/patch/11469701/ > > The above patch has been accepted. However, the variable pp->mem_size > in dw_pcie_host_init() is still a u32 whereas the value returned by > resource_size() is u64. If the resource size has non-zero upper 32-bits, > those upper 32-bits will be lost when assigning: > pp->mem_size = resource_size(pp->mem). > > Since current callers seem happy with the existing 4GB implementation > and fixing the u32 limit is beyond my available resources and has a long > high-impact tail, a warning seemed to be a good choice to highlight > this issue in case someone else decides to map a MEM region that is > greater than 4GB. > > Removing the warning will avoid such discussions. Without this warning, > this limitation will go unnoticed and will only impact whoever has to > deal with it. It cost me time to figure it out when I had an application > that needed a region larger than 4GB. I figured the most I could do about > it is to raise the issue by adding a warning. You did the right thing (and you helped me unearth some major deficiencies in current DWC code). Unfortunately I have to drop: 9e73fa02aa00 ("PCI: dwc: Warn if MEM resource size exceeds max for 32-bits") because it triggers regressions (and it is still not in the mainline, IMO there would be more if we send it upstream). I will keep: e1fc129219a8 ("PCI: dwc: Program outbound ATU upper limit register") because it is a step in the right direction and makes sense on its own. Thanks for all the effort you put into this. Lorenzo > Regards, > Alan > > > > > >
