Re: [PATCH v4 14/17] x86/asm/entry/64: Migrate error and interrupt exit work to C

Andy Lutomirski
2015-07-03T16:24:43Z

On Fri, Jul 3, 2015 at 7:37 AM, Denys Vlasenko <vda.linux@googlemail.com> wrote:
> On Thu, Jul 2, 2015 at 6:09 PM, Andy Lutomirski <luto@amacapital.net> wrote:
>> On Thu, Jul 2, 2015 at 5:09 AM, Borislav Petkov <bp@alien8.de> wrote:
>>> On Mon, Jun 29, 2015 at 12:33:46PM -0700, Andy Lutomirski wrote:
>>>> Signed-off-by: Andy Lutomirski <luto@kernel.org>
>>>> ---
>>>> arch/x86/entry/entry_64.S | 63 +++++++++++-----------------------------
>>>> arch/x86/entry/entry_64_compat.S | 5 ++++
>>>> 2 files changed, 22 insertions(+), 46 deletions(-)
>>>>
>>>> diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
>>>> index ce2c9049abef..08a37ec049f0 100644
>>>> --- a/arch/x86/entry/entry_64.S
>>>> +++ b/arch/x86/entry/entry_64.S
>>>> @@ -508,7 +508,16 @@ END(irq_entries_start)
>>>>
>>>> testb $3, CS(%rsp)
>>>> jz 1f
>>>> +
>>>> + /*
>>>> + * IRQ from user mode. Switch to kernel gsbase and inform context
>>>> + * tracking that we're in kernel mode.
>>>> + */
>>>> SWAPGS
>>>> +#ifdef CONFIG_CONTEXT_TRACKING
>>>> + call enter_from_user_mode
>>>> +#endif
>>>
>>> I think you can make this much cleaner by getting rid of the ifdeffery
>>> and pushing it into the enter_from_user_mode() function:
>>>
>>> __visible void enter_from_user_mode(void)
>>> {
>>> #ifdef CONFIG_CONTEXT_TRACKING
>>>
>>> ...
>>>
>>> #endif
>>> }
>>>
>>> The disadvantage of all that cleanliness is that we get one dumb
>>>
>>> call enter_from_user_mode
>>>
>>> there to this abomination:
>>>
>>> ffffffff810014f0 <enter_from_user_mode>:
>>> ffffffff810014f0: e8 db 97 67 00 callq ffffffff8167acd0 <__fentry__>
>>> ffffffff810014f5: 55 push %rbp
>>> ffffffff810014f6: 48 89 e5 mov %rsp,%rbp
>>> ffffffff810014f9: 5d pop %rbp
>>> ffffffff810014fa: c3 retq
>>> ffffffff810014fb: 0f 1f 44 00 00 nopl 0x0(%rax,%rax,1)
>>>
>>> which sux.
>>>
>>> We sure could use LTO here.
>>>
>>
>> You mean link-time asm optimizations? Turning off frame pointers in
>> leaf functions as long as rbp is still preserved might not be so
>> terrible either.
>>
>> I'm torn on this one. In principle, you're right, or we could have a
>> macro CALL_ENTER_FROM_USER_MODE that does nothing if context tracking
>> is off. OTOH, that's also kind of messy.
>
> busybox has IF() macros:
>
> IF_CONTEXT_TRACKING(call enter_from_user_mode)
>
> Unlike #if, this needs one line, not three.

I predict all of these call sites will get moved to C before 4.3. If
not, then I'd be glad to clean this part up.

--Andy