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

[Denys Vlasenko]

On Mon, Mar 9, 2015 at 9:11 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>> @@ -253,11 +247,13 @@ GLOBAL(system_call_after_swapgs)
>>          */
>>         ENABLE_INTERRUPTS(CLBR_NONE)
>>         ALLOC_PT_GPREGS_ON_STACK 8              /* +8: space for orig_ax */
>> +       movq    %rcx,RIP(%rsp)
>> +       movq    PER_CPU_VAR(old_rsp),%rcx
>> +       movq    %r11,EFLAGS(%rsp)
>> +       movq    %rcx,RSP(%rsp)
>> +       movq_cfi rax,ORIG_RAX
>>         SAVE_C_REGS_EXCEPT_RAX_RCX_R11
>>         movq    $-ENOSYS,RAX(%rsp)
>> -       movq_cfi rax,ORIG_RAX
>> -       movq    %r11,EFLAGS(%rsp)
>> -       movq    %rcx,RIP(%rsp)
>
> Why the reordering?

No strong reason.

iret stack is "above" the rest of pt_regs.

This does not matter now, but when/if we convert to PUSHes
for register saving, pushes which build iret frame
will have to be before "save C-clobbered registers" part,
exactly as in this patch.
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