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

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Date:	Fri, 3 Jul 2015 09:24:14 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Denys Vlasenko <vda.linux@...glemail.com>
Cc:	Borislav Petkov <bp@...en8.de>, Andy Lutomirski <luto@...nel.org>,
	X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Frédéric Weisbecker <fweisbec@...il.com>,
	Rik van Riel <riel@...hat.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Kees Cook <keescook@...omium.org>,
	Brian Gerst <brgerst@...il.com>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH v4 14/17] x86/asm/entry/64: Migrate error and interrupt
 exit work to C

On Fri, Jul 3, 2015 at 7:37 AM, Denys Vlasenko <vda.linux@...glemail.com> wrote:
> On Thu, Jul 2, 2015 at 6:09 PM, Andy Lutomirski <luto@...capital.net> wrote:
>> On Thu, Jul 2, 2015 at 5:09 AM, Borislav Petkov <bp@...en8.de> wrote:
>>> On Mon, Jun 29, 2015 at 12:33:46PM -0700, Andy Lutomirski wrote:
>>>> Signed-off-by: Andy Lutomirski <luto@...nel.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
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