Re: [PATCH v8 1/3] x86: Expand exception table to allow new handling options

[Brian Gerst]

On Fri, Jan 8, 2016 at 10:39 PM, Brian Gerst <brgerst@xxxxxxxxx> wrote:
> On Fri, Jan 8, 2016 at 8:52 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>> On Fri, Jan 8, 2016 at 12:49 PM, Tony Luck <tony.luck@xxxxxxxxx> wrote:
>>> Huge amounts of help from  Andy Lutomirski and Borislav Petkov to
>>> produce this. Andy provided the inspiration to add classes to the
>>> exception table with a clever bit-squeezing trick, Boris pointed
>>> out how much cleaner it would all be if we just had a new field.
>>>
>>> Linus Torvalds blessed the expansion with:
>>>   I'd rather not be clever in order to save just a tiny amount of space
>>>   in the exception table, which isn't really criticial for anybody.
>>>
>>> The third field is a simple integer indexing into an array of handler
>>> functions (I thought it couldn't be a relative pointer like the other
>>> fields because a module may have its ex_table loaded more than 2GB away
>>> from the handler function - but that may not be actually true. But the
>>> integer is pretty flexible, we are only really using low two bits now).
>>>
>>> We start out with three handlers:
>>>
>>> 0: Legacy - just jumps the to fixup IP
>>> 1: Fault - provide the trap number in %ax to the fixup code
>>> 2: Cleaned up legacy for the uaccess error hack
>>
>> I think I preferred the relative function pointer approach.
>>
>> Also, I think it would be nicer if the machine check code would invoke
>> the handler regardless of which handler (or class) is selected.  Then
>> the handlers that don't want to handle #MC can just reject them.
>>
>> Also, can you make the handlers return bool instead of int?
>
> I'm hashing up an idea that could eliminate alot of text in the .fixup
> section, but it needs the integer handler method to work.  We have
> alot of fixup code that does "mov $-EFAULT, reg; jmp xxxx".  If we
> encode the register in the third word, the handler can be generic and
> no fixup code for each user access would be needed.  That would
> recover alot of the memory used by expanding the exception table.

On second thought, this could still be implemented with a relative
function pointer.  We'd just need a separate function for each
register.

--
Brian Gerst