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. -- 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/
