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? --Andy -- Andy Lutomirski AMA Capital Management, LLC -- 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/
