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 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>