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.
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-- Sincerely, Yeoreum Yun