Re: [tip:x86/asm] x86/asm/entry/64: Migrate error and IRQ exit work to C and remove old assembly code

[Andy Lutomirski]

On Tue, Aug 11, 2015 at 3:18 PM, Frederic Weisbecker <fweisbec@xxxxxxxxx> wrote:
> On Tue, Jul 07, 2015 at 03:53:29AM -0700, tip-bot for Andy Lutomirski wrote:
>> Commit-ID:  02bc7768fe447ae305e924b931fa629073a4a1b9
>> Gitweb:     http://git.kernel.org/tip/02bc7768fe447ae305e924b931fa629073a4a1b9
>> Author:     Andy Lutomirski <luto@xxxxxxxxxx>
>> AuthorDate: Fri, 3 Jul 2015 12:44:31 -0700
>> Committer:  Ingo Molnar <mingo@xxxxxxxxxx>
>> CommitDate: Tue, 7 Jul 2015 10:59:08 +0200
>>
>> x86/asm/entry/64: Migrate error and IRQ exit work to C and remove old assembly code
>>
>> Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
>> Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
>> Cc: Borislav Petkov <bp@xxxxxxxxx>
>> Cc: Brian Gerst <brgerst@xxxxxxxxx>
>> Cc: Denys Vlasenko <dvlasenk@xxxxxxxxxx>
>> Cc: Denys Vlasenko <vda.linux@xxxxxxxxxxxxxx>
>> Cc: Frederic Weisbecker <fweisbec@xxxxxxxxx>
>> Cc: H. Peter Anvin <hpa@xxxxxxxxx>
>> Cc: Kees Cook <keescook@xxxxxxxxxxxx>
>> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
>> Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
>> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
>> Cc: Rik van Riel <riel@xxxxxxxxxx>
>> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
>> Cc: paulmck@xxxxxxxxxxxxxxxxxx
>> Link: http://lkml.kernel.org/r/60e90901eee611e59e958bfdbbe39969b4f88fe5.1435952415.git.luto@xxxxxxxxxx
>> Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
>> ---
>>  arch/x86/entry/entry_64.S        | 64 +++++++++++-----------------------------
>>  arch/x86/entry/entry_64_compat.S |  5 ++++
>>  2 files changed, 23 insertions(+), 46 deletions(-)
>>
>> diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
>> index 83eb63d..168ee26 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
>
> There have been a lot of patches going there lately so I couldn't follow
> everything and since you just started a discussion on context tracking, I
> just had a look on the latest change.
>
> So it seems we're now calling user_exit() on IRQ entry. This is not something
> we want. We already have everything we need with rcu_irq_enter() and
> vtime_account_irq_enter(). user_exit() brings a lot of overhead here that we
> don't need. Plus this is called unconditionally since CONFIG_CONTEXT_TRACKING=y
> on most distros now.
>
> We really want the context tracking code to be called on syscall slow path only
> (and exceptions with static keys but an exception slow path would be desired as well).

Can you explain to me what context tracking does that rcu_irq_enter
and vtime_account_irq_enter don't do that's expensive?  Frankly, I'd
rather drop everything except the context tracking callback.

We also need this for the deletion of exception_enter from the trap
entries to be correct.

Like I said in the other thread, there are too many hooks for arch
code to juggle.  Grumble.

--Andy
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