Re:Linux page table isolation is not needed on AMD processors
by revelation on Wednesday January 03, 2018 @07:00PM
(#16066703)
It's counter intuitive at first, but the key to understand how this works is that while you can use an MMU to assign chunks of physical memory to a process, you can of course also just use the MMU to assign the memory mapped IO registers of say a PCI express peripheral to a process.
That is in a nutshell what a "userland driver" is. It's not too far removed from poking the parallel port at 0x378 on your DOS computer :)
Re:Linux page table isolation is not needed on AMD processors
by arghwhat on Saturday January 06, 2018 @07:26AM
(#16084985)
A user-space driver doesn't communicate with the kernel. It is assigned DMA buffers, and communicates with the NIC solely through reading and writing to shared memory buffers.
Even before this fix, the benefits were massive, as sending a buffer was just writing to some memory, rather than syscalls and copies galore.
I would personally think that is worse, though please correct me if I'm wrong. The userland driver will run with an isolated PT like any other userland process won't it? If so, it will suffer the same slowdown that every other process now has every-time it has to communicate with the kernel, which I would think would be a lot for a driver.