Re: [PATCH 07/18] x86/asm/64: Merge the fast and slow SYSRET paths

Andy Lutomirski
2017-11-01T20:35:35Z

On Wed, Nov 1, 2017 at 10:25 AM, Brian Gerst <brgerst@gmail.com> wrote:
> On Thu, Oct 26, 2017 at 4:26 AM, Andy Lutomirski <luto@kernel.org> wrote:
>> They did almost the same thing. Remove a bunch of pointless
>> instructions (mostly hidden in macros) and reduce cognitive load by
>> merging them.
>>
>> Signed-off-by: Andy Lutomirski <luto@kernel.org>
>> ---
>> arch/x86/entry/entry_64.S | 6 +++---
>> 1 file changed, 3 insertions(+), 3 deletions(-)
>>
>> diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
>> index 9dafafa3e0ec..c855ee91a3a5 100644
>> --- a/arch/x86/entry/entry_64.S
>> +++ b/arch/x86/entry/entry_64.S
>> @@ -220,10 +220,9 @@ entry_SYSCALL_64_fastpath:
>> TRACE_IRQS_ON /* user mode is traced as IRQs on */
>> movq RIP(%rsp), %rcx
>> movq EFLAGS(%rsp), %r11
>> - RESTORE_C_REGS_EXCEPT_RCX_R11
>> - movq RSP(%rsp), %rsp
>> + addq $6*8, %rsp /* skip extra regs -- they were preserved */
>> UNWIND_HINT_EMPTY
>> - USERGS_SYSRET64
>> + jmp .Lpop_c_regs_except_rcx_r11_and_sysret
>>
>> 1:
>> /*
>> @@ -315,6 +314,7 @@ syscall_return_via_sysret:
>> /* rcx and r11 are already restored (see code above) */
>> UNWIND_HINT_EMPTY
>> POP_EXTRA_REGS
>> +.Lpop_c_regs_except_rcx_r11_and_sysret:
>> popq %rsi /* skip r11 */
>> popq %r10
>> popq %r9
>
> Wouldn't it be more logical to keep the SYSRET path at the end of the
> fast path (reverse of what you are doing here)? That way the fast
> path falls through without jumping.

Ugh, probably. I think I'll save that for v3, though. I want to keep
this moving.