Re: [PATCH] x86/entry/64: Remove duplicate syscall table for fast path

[Brian Gerst]

On Wed, Dec 9, 2015 at 1:53 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> On Wed, Dec 9, 2015 at 5:02 AM, Brian Gerst <brgerst@xxxxxxxxx> wrote:
>> Instead of using a duplicate syscall table for the fast path, create stubs for
>> the syscalls that need pt_regs that save the extra registers if a flag for the
>> slow path is not set.
>>
>> Signed-off-by: Brian Gerst <brgerst@xxxxxxxxx>
>> To: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
>> Cc: Andy Lutomirski <luto@xxxxxxxxxx>
>> Cc: the arch/x86 maintainers <x86@xxxxxxxxxx>
>> Cc: Linux Kernel Mailing List <linux-kernel@xxxxxxxxxxxxxxx>
>> Cc: Borislav Petkov <bp@xxxxxxxxx>
>> Cc: FrÃdÃric Weisbecker <fweisbec@xxxxxxxxx>
>> Cc: Denys Vlasenko <dvlasenk@xxxxxxxxxx>
>> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
>> ---
>>
>> Applies on top of Andy's syscall cleanup series.
>
> A couple questions:
>
>> @@ -306,15 +306,37 @@ END(entry_SYSCALL_64)
>>
>>  ENTRY(stub_ptregs_64)
>>         /*
>> -        * Syscalls marked as needing ptregs that go through the fast path
>> -        * land here.  We transfer to the slow path.
>> +        * Syscalls marked as needing ptregs land here.
>> +        * If we are on the fast path, we need to save the extra regs.
>> +        * If we are on the slow path, the extra regs are already saved.
>>          */
>> -       DISABLE_INTERRUPTS(CLBR_NONE)
>> -       TRACE_IRQS_OFF
>> -       addq    $8, %rsp
>> -       jmp     entry_SYSCALL64_slow_path
>> +       movq    PER_CPU_VAR(cpu_current_top_of_stack), %r10
>> +       testl   $TS_SLOWPATH, ASM_THREAD_INFO(TI_status, %r10, 0)
>> +       jnz     1f
>
> OK (but see below), but why not do:
>
> addq $8, %rsp
> jmp entry_SYSCALL64_slow_path

I've always been adverse to doing things like that because it breaks
call/return branch prediction.
Also, are there any side effects to calling enter_from_user_mode()
more than once?

> here instead of the stack munging below?
>
>> +       subq    $SIZEOF_PTREGS, %r10
>> +       SAVE_EXTRA_REGS base=r10
>> +       movq    %r10, %rbx
>> +       call    *%rax
>> +       movq    %rbx, %r10
>> +       RESTORE_EXTRA_REGS base=r10
>> +       ret
>> +1:
>> +       jmp     *%rax
>>  END(stub_ptregs_64)

After some thought, that can be simplified.  It's only executed on the
fast path, so pt_regs is at 8(%rsp).

> Also, can we not get away with keying off rip or rsp instead of
> ti->status?  That should be faster and less magical IMO.

Checking if the return address is the instruction after the fast path
dispatch would work.

Simplified version:
ENTRY(stub_ptregs_64)
    cmpl $fast_path_return, (%rsp)
    jne 1f
    SAVE_EXTRA_REGS offset=8
    call *%rax
    RESTORE_EXTRA_REGS offset=8
    ret
1:
    jmp *%rax
END(stub_ptregs_64)

--
Brian Gerst
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