Hi Arnd. > On Thu, Nov 6, 2025, at 10:40, Yeoreum Yun wrote: > > apply FEAT_LSUI instruction to emulate deprecated swpX instruction. > > Can you explain in the changrelog why you do this? > > In particular, is this a performance optimization or is this required > for correctness in some scenario?
I think the main purpose using for this is to "remove the toggling the PSTATE.PAN" bit. so that remove some *potential* problem which can happen when PAN bit is cleared. also, though swpb might add some complexity, but swp can get a little bit of benifit by removing the ll/sc way and reduce the amount of instructions. > I would have expected that there is very little benefit in > changing the swp/swpb emulation here if the existing code > has to remain compiled into the kernel and the overhead of > the trap is so much larger than the uaccess_enable_privileged() > overhead. Yeap. since the *trap* overhead is much larger than the uaccess_enable_privileged() in the performnace perspecitve, It would has a little effect. However, as I mention above, the main purpose of this is to remove toggling the PSTATE.PAN if possible. there would be a little bit of additional commplexity for swpb but it seems affordable. Thanks. [...] -- Sincerely, Yeoreum Yun
