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OTOH, disabling IBRS around idle takes two > >> MSR writes, which will increase the idle latency. > >> > >> Therefore, only disable IBRS around deeper idle states. Shallow idle > >> states are bounded by the tick in duration, since NOHZ is not allowed > >> for them by virtue of their short target residency. > >> > >> Only do this for mwait-driven idle, since that keeps interrupts disabled > >> across idle, which makes disabling IBRS vs IRQ-entry a non-issue. > >> > >> Note: C6 is a random threshold, most importantly C1 probably shouldn't > >> disable IBRS, benchmarking needed. > >> > >> Suggested-by: Tim Chen > >> Signed-off-by: Peter Zijlstra (Intel) > >> Signed-off-by: Borislav Petkov > >> Reviewed-by: Josh Poimboeuf > >> Signed-off-by: Borislav Petkov > >> Origin: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git bf5835bcdb96 > >> Signed-off-by: Jan Beulich > > > > Acked-by: Roger Pau Monné > > Thanks. > > > One unrelated comment below. > > [...] > >> @@ -932,8 +939,6 @@ static void cf_check mwait_idle(void) > >> pm_idle_save(); > >> else > >> { > >> - struct cpu_info *info = get_cpu_info(); > >> - > >> spec_ctrl_enter_idle(info); > >> safe_halt(); > >> spec_ctrl_exit_idle(info); > > > > Do we need to disable speculation just for the hlt if there's no > > C state change? > > > > It would seem to me like the MSR writes could add a lot of latency > > when there's no C state change. > > HLT enters (at least) C1, so is a C-state change to me as well. Plus > we may remain there for a while, and during that time we'd like to > not unduly impact the other thread. OK, but it's not a "deeper C state" as mentioned in the commit message. Thanks, Roger.