Re: [PATCH] x86/irq: Preserve vector in orig_ax for APIC code

[Alexander Graf <graf@amazon.com>]

On 26.08.20 16:27, Thomas Gleixner wrote:
> 
> On Wed, Aug 26 2020 at 13:53, Alexander Graf wrote:
>> Commit 633260fa143 ("x86/irq: Convey vector as argument and not in ptregs")
>> changed the handover logic of the vector identifier from ~vector in orig_ax
>> to purely register based. Unfortunately, this field has another consumer
>> in the APIC code which the commit did not touch. The net result was that
>> IRQ balancing did not work and instead resulted in interrupt storms, slowing
>> down the system.
> 
> The net result is an observationof the symptom but that does not explain
> what the underlying technical issue is.
> 
>> This patch restores the original semantics that orig_ax contains the vector.
>> When we receive an interrupt now, the actual vector number stays stored in
>> the orig_ax field which then gets consumed by the APIC code.
>>
>> To ensure that nobody else trips over this in the future, the patch also adds
>> comments at strategic places to warn anyone who would refactor the code that
>> there is another consumer of the field.
>>
>> With this patch in place, IRQ balancing works as expected and performance
>> levels are restored to previous levels.
> 
> There's a lot of 'This patch and we' in that changelog. Care to grep
> for 'This patch' in Documentation/process/ ?
> 
>> diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
>> index df8c017..22e829c 100644
>> --- a/arch/x86/entry/entry_32.S
>> +++ b/arch/x86/entry/entry_32.S
>> @@ -727,7 +727,7 @@ SYM_CODE_START_LOCAL(asm_\cfunc)
>>        ENCODE_FRAME_POINTER
>>        movl    %esp, %eax
>>        movl    PT_ORIG_EAX(%esp), %edx         /* get the vector from stack */
>> -     movl    $-1, PT_ORIG_EAX(%esp)          /* no syscall to restart */
>> +     /* keep vector on stack for APIC's irq_complete_move() */
> 
> Yes that's fixing your observed wreckage, but it introduces a worse one.
> 
> user space
>    -> interrupt
>         push vector into orig_ax (values are in the ranges of 0-127 and -128 - 255
>                                   except for the system vectors which do
>                                   not go through this code)
>        handle()
>        ...
>        exit_to_user_mode_loop()
>           arch_do_signal()
>              /* Did we come from a system call? */
>              if (syscall_get_nr(current, regs) >= 0) {
> 
>                 ---> BOOM for any vector 0-127 because syscall_get_nr()
>                           resolves to regs->orig_ax
> 
> Going to be fun to debug.

Hah, that's the code flow I was looking for to understand why the value 
was negative in the first place. Thanks a lot for pointing it out!

> 
> The below nasty hack cures it, but I hate it with a passion. I'll look
> deeper for a sane variant.

An alternative (that doesn't make the code easier to read, but would fix 
the issue at hand) would be touse a pushq imm16 with vector | 0x8000 
instead to always make the value negative, no?


Alex



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