I need to provide a huge circular buffer (a few GB) for the bus-mastering DMA PCIe device implemented in FPGA.
The buffers should not be reserved at the boot time. Therefore, the buffer may be not contiguous.
The device supports scatter-gather (SG) operation, but for performance reasons, the addresses and lengths of consecutive contiguous segments of the buffer are stored inside the FPGA. Therefore, usage of standard 4KB pages is not acceptable (there would be up to 262144 segments for each 1GB of the buffer).
The right solution should allocate the buffer consisting of 2MB hugepages in the user space (reducing the maximum number of segments by factor of 512). The virtual address of the buffer should be transferred to the kernel driver via ioctl. Then the addresses and the length of the segments should be calculated and written to the FPGA.
In theory, I could use get_user_pages to create the list of the pages, and then call sg_alloc_table_from_pages to obtain the SG list suitable to program the DMA engine in FPGA. Unfortunately, in this approach I must prepare the intermediate list of page structures with length of 262144 pages per 1GB of the buffer. This list is stored in RAM, not in the FPGA, so it is less problematic, but anyway it would be good to avoid it.
In fact I don't need to keep the pages maped for the kernel, as the hugepages are protected against swapping out, and they are mapped for the user space application that will process the received data.
So what I'm looking for is a function
sg_alloc_table_from_user_hugepages, that could take such a user-space address of the hugepages-based memory buffer, and transfer it directly into the right scatterlist, without performing unnecessary and memory-consuming mapping for the kernel. Of course such a function should verify that the buffer indeed consists of hugepages.
Thank you in advance,
with best regards,
PS. I have also asked that question on stackoverflow. However, maybe here is a bigger chance to find somebody who have faced and successfully solved such problem.