Re: [PATCH 3/4] x86: save user rsp in pt_regs->sp on SYSCALL64 fastpath

From: Denys Vlasenko
Date: Tue Mar 10 2015 - 10:00:52 EST

On Tue, Mar 10, 2015 at 2:26 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> 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.
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