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Re: [cfe-dev] RFC: Support x86 interrupt and exception handlers


On Mon, Sep 21, 2015 at 11:52 AM, John Criswell <jtcriswel@gmail.com> wrote:
> On 9/21/15 12:27 PM, H.J. Lu via cfe-dev wrote:
>>
>> On Thu, Sep 17, 2015 at 12:26 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
>>>
>>> On Tue, Sep 15, 2015 at 1:11 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
>>>>>
>>>>> To implement interrupt and exception handlers for x86 processors, a
>>>>> compiler should support:
>>>>>
>>>>> 1. void * __builtin_ia32_interrupt_data (void)
>>>>
>>>> I got a feedback on the name of this builtin function.  Since
>>>> it also works for 64-bit,  we should avoid ia32 in its name.
>>>> We'd like to change it to
>>>>
>>>> void * __builtin_interrupt_data (void)
>>>>
>>> Here is the updated spec.
>>>
>> This updated spec adds
>>
>>     unsigned int __builtin_exception_error (void)
>>     unsigned long long int __builtin_exception_error (void)
>>
>> This function returns the exception error code pushed onto the stack by
>> processor.  Its return value is 64 bits in 64-bit mode and 32 bits in
>> 32-bit mode.  This function can only be used in exception handler.
>
>
> Exception handlers can, in general, call regular functions which, in turn,
> might want to access the error code.  Given that operating system kernels
> are always entered via an interrupt, trap, or system call, there should
> always be an error code available (on x86, non-error-code interrupts can
> just make up an error code).
>
>>
>> It also changes the definition of
>>
>> void * __builtin_interrupt_data (void)
>>
>> so that it returns a pointer to the data layout pushed onto stack
>> by processor for both interrupt and exception handlers.
>>
>>
>
> You might want to have a look at Secure Virtual Architecture (SVA). One of

I believe my x86 interrupt attribute is unrelated to SVA.

> If the implementation is useful, SVA is publicly available at
> https://github.com/jtcriswell/SVA.
>
> Finally, to echo Joerg's concerns, it's not clear that having
> exception/interrupt handlers declared as a special type is really helpful.
> It's not immediately obvious that you get a benefit from doing that vs.
> doing what most system software does (having assembly code that saves
> processor state and calls a C function).  I think you should do some
> experiments to demonstrate the benefit that one can get with your method to
> see if it is worth adding complexity to the compiler.
>

The main purpose of x86 interrupt attribute is to allow programmers
to write x86 interrupt/exception handlers in C WITHOUT assembly
stubs to avoid extra branch from assembly stubs to C functions.  I
want to keep the number of new intrinsics to minimum without sacrificing
handler performance. I leave faking error code in interrupt handler to
the programmer.

-- 
H.J.


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