On Tue, Mar 10, 2015 at 3:02 PM, Andy Lutomirski <[email protected]> wrote: > On Tue, Mar 10, 2015 at 7:00 AM, Denys Vlasenko > <[email protected]> wrote: >> On Tue, Mar 10, 2015 at 2:26 PM, Andy Lutomirski <[email protected]> wrote: >>> usersp is IMO tolerable. The nasty thing is the FIXUP_TOP_OF_STACK / >>> RESTORE_TOP_OF_STACK garbage, and this patch is the main step toward >>> killing that off completely. I've still never convinced myself that >>> there aren't ptrace-related info leaks in there. >>> >>> Denys, did you ever benchmark what happens if we use push instead of >>> mov? I bet that we get that cycle back and more, not to mention much >>> less icache usage. >> >> Yes, I did. >> Push conversion seems to perform the same as current, MOV-based code. >> >> The expected win there that we lose two huge 12-byte insns >> which store __USER_CS and __USER_DS in iret frame. >> >> MOVQ imm,ofs(%rsp) has a very unfortunate encoding in x86: >> - needs REX prefix >> - no sing-extending imm8 form exists for it >> - ofs in our case can't fit into 8 bits >> - (%esp) requires SIB byte >> >> In my tests, each such instruction adds one cycle. >> >> Compare this to PUSH imm8, which is 2 bytes only. > > Does that mean that using push on top of this patch gets us our cycle back?
Maybe. I can't be sure about it. In general I see a jitter of 1-2, sometimes 3 cycles even when I do changes which merely change code size (e.g. replacing equivalent insns). This may be caused by jump targets getting aligned differently wrt cacheline boundaries. If second/third/fourth insn after current one is not fetched because it did not fit into the cacheline, then some insn decoders don't get anything to chew on. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
