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

[Andy Lutomirski]

On Tue, Mar 10, 2015 at 7:00 AM, Denys Vlasenko
<vda.linux@xxxxxxxxxxxxxx> wrote:
> 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.

Does that mean that using push on top of this patch gets us our cycle back?

--Andy

-- 
Andy Lutomirski
AMA Capital Management, LLC
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