On 25.05.2022 16:33, Andrew Cooper wrote: > On 25/05/2022 12:14, Jan Beulich wrote: >> On 25.05.2022 12:52, Andrew Cooper wrote: >>> On 25/05/2022 09:13, Roger Pau Monne wrote: >>>> Rename the flag to better note that it's not actually forcing any IPIs >>>> to be issued if none is required, but merely avoiding the usage of TLB >>>> flush assistance (which itself can avoid the sending of IPIs to remote >>>> processors). >>>> >>>> No functional change expected. >>>> >>>> Requested-by: Jan Beulich <[email protected]> >>>> Signed-off-by: Roger Pau Monné <[email protected]> >>>> --- >>>> Changes since v2: >>>> - New in this version. >>> :( This needs reverting. >>> >>> It is specific to IPIs, because of our current choice of algorithm for >>> freeing pagetables. >>> >>> "no assist" excludes ipi-helper hypercalls which invoke >>> INVALIDATE_TLB_VECTOR. Such hypercalls do exist and specifically would >>> be improvement that we ought to use. >>> >>> Furthermore, we do want to work around the limitation that created >>> FLUSH_FORCE_IPI, because we absolutely do want to be able to use >>> hypercalls/remote TLB flushing capabilities when available. >>> >>> I accept that FORCE_IPI perhaps isn't a perfect name, but it's a whole >>> lot less bad than NO_ASSIST. >> But FORCE_IPI has caused actual confusion while reviewing patch 2. >> If NO_ASSIST doesn't suit you and FORCE_IPI is also wrong, can you >> suggest a better name fitting both aspects? > > I don't actually agree that FORCE_IPI is unclear to begin with.
You did see the earlier communication with Roger, didn't you? To me the name pretty clearly suggests "always IPI" (hence "force"), i.e. ... > The safety property required is "if you need to contact a remote CPU, it > must be by IPI to interlock with Xen's #PF handler". ... not in any way limited to remote CPUs. Yet patch 2 is about cases where things are safe because no IPI will be needed (not even a self-IPI). > FORCE_IPI is very close to meaning this. If anything else is unclear, > it can be clarified in the adjacent comment. I'm afraid I don't think a comment is what would help here. Jan
