Am 01.06.2015 um 13:34 schrieb Paolo Bonzini: > > > On 01/06/2015 09:47, Christian Borntraeger wrote: >> >> 1: "disable", "guest", "disable again and save", "restore to disable", "enable" >> and now it is >> 2: "disable", "guest", "enable" >> and with your patch it is >> 3: "disable", "guest", "enable", "disable, "enable" >> >> I assume that 3 and 1 are similar in its costs, so this is probably ok. > > At least on x86, 3 and 2 are similar, but 3 is much more expensive than > 1! See https://lkml.org/lkml/2015/5/5/835: That does not make sense. If 3 and 2 are similar, then 2 must be much more expensive than 1 as well. As 2 is a strict subset of 1 it must be cheaper, no? > > Cost of: CLI insn same-IF : 0 cycles > Cost of: CLI insn flip-IF : 0 cycles > Cost of: STI insn same-IF : 0 cycles > Cost of: STI insn flip-IF : 0 cycles > Cost of: PUSHF insn : 0 cycles > Cost of: POPF insn same-IF : 20 cycles > Cost of: POPF insn flip-IF : 28 cycles > Cost of: local_irq_save() fn : 20 cycles > Cost of: local_irq_restore() fn same-IF : 24 cycles > Cost of: local_irq_restore() fn flip-IF : 28 cycles > Cost of: irq_save()+restore() fn same-IF : 48 cycles > Cost of: irq_save()+restore() fn flip-IF : 48 cycles Yes its similar on s390. local_irq_save/restore is noticable in guest exit hot loops (thats what inspired my patch), but a simple irq disable is just single cycle pipelined. Given the design of aggressive out-out order designs with all the architectural ordering this makes sense. Christian -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
