Re: [PATCH] x86 interrupt attribute

["H.J. Lu" <hjl dot tools at gmail dot com>]

On Sun, Oct 4, 2015 at 2:07 PM, Uros Bizjak <ubizjak@gmail.com> wrote:
> On Sun, Oct 4, 2015 at 10:51 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
>> On Sun, Oct 4, 2015 at 1:00 PM, Uros Bizjak <ubizjak@gmail.com> wrote:
>>> On Sun, Oct 4, 2015 at 8:15 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
>>>
>>>>> Looking a bit deeper into the code, it looks that we want to realign
>>>>> the stack in the interrupt handler. Let's assume that interrupt
>>>>> handler is calling some other function that saves SSE vector regs to
>>>>> the stack. According to the x86 ABI, incoming stack of the called
>>>>> function is assumed to be aligned to 16 bytes. But, interrupt handler
>>>>> violates this assumption, since the stack could be aligned to only 4
>>>>> bytes for 32bit and 8 bytes for 64bit targets. Entering the called
>>>>> function with stack, aligned to less than 16 bytes will certainly
>>>>> violate ABI.
>>>>>
>>>>> So, it looks to me that we need to realign the stack in the interrupt
>>>>> handler unconditionally to 16bytes. In this case, we also won't need
>>>>> the following changes:
>>>>>
>>>>
>>>> Current stack alignment implementation requires at least
>>>> one, maybe two, scratch registers:
>>>>
>>>> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67841
>>>>
>>>> Extend it to the interrupt handler, which doesn't have any scratch
>>>> registers may require significant changes in backend as well as
>>>> register allocator.
>>>
>>> But without realignment, the handler is unusable for anything but
>>> simple functions. The handler will crash when called function will try
>>> to save vector reg to stack.
>>>
>>
>> We can use unaligned load and store to avoid crash.
>
> Oh, sorry, I meant "called function will crash", like:
>
> -> interrupt when %rsp = 0x...8 ->
> -> interrupt handler ->
> -> calls some function that tries to save xmm reg to stack
> -> crash in the called function
>

It should be fixed by this patch.   But we need to fix stack
alignment in interrupt handler to avoid scratch register.


-- 
H.J.
---
commit 15f48be1dc7ff48207927d0b835e593d058f695b
Author: H.J. Lu <hjl.tools@gmail.com>
Date:   Sun Oct 4 16:14:03 2015 -0700

    Correctly set incoming stack boundary for interrupt handler

diff --git a/gcc/config/i386/i386.c b/gcc/config/i386/i386.c
index 7ebdcd9..0f0cc3c 100644
--- a/gcc/config/i386/i386.c
+++ b/gcc/config/i386/i386.c
@@ -12037,8 +12037,11 @@ ix86_minimum_incoming_stack_boundary (bool sibcall)
 {
   unsigned int incoming_stack_boundary;

+  /* Stack of interrupt handler is always aligned to word_mode.  */
+  if (cfun->machine->func_type != TYPE_NORMAL)
+    incoming_stack_boundary = TARGET_64BIT ? 64 : 32;
   /* Prefer the one specified at command line. */
-  if (ix86_user_incoming_stack_boundary)
+  else if (ix86_user_incoming_stack_boundary)
     incoming_stack_boundary = ix86_user_incoming_stack_boundary;
   /* In 32bit, use MIN_STACK_BOUNDARY for incoming stack boundary
      if -mstackrealign is used, it isn't used for sibcall check and