On 2024-08-01 20:32, Kasireddy, Vivek wrote:

Hi Huan,

...

This patchset attempts to fix some errors in udmabuf and remove the upin_list structure.

Some of this fix just gather the patches which I upload before.

Patch1

Try to remove page fault mmap and direct map it. Due to current udmabuf has already obtained and pinned the folio upon completion of the creation.This means that the physical memory has already been acquired, rather than being accessed dynamically. The current page fault method only saves some page table memory.

As a result, the page fault mechanism has lost its purpose as a demanding page. Due to the fact that page fault requires trapping into kernel mode and filling in when accessing the corresponding virtual address in mmap, this means that user mode access to virtual addresses needs to trap into kernel mode.

Therefore, when creating a large size udmabuf, this represents a considerable overhead.

Just want to mention that for the main use-case the udmabuf driver is designed for, (sharing Qemu Guest FB with Host for GPU DMA), udmabufs are not created very frequently. And, I think providing CPU access via mmap is just a backup, mainly intended for debugging purposes.

FYI, Mesa now uses udmabuf for supporting dma-bufs with software rendering.