[Bug target/113321] New: x86-64: Make __attribute__((interrupt)) more robust

[Bug target/113321] New: x86-64: Make __attribute__((interrupt)) more robust

[hpa at zytor dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113321

            Bug ID: 113321
           Summary: x86-64: Make __attribute__((interrupt)) more robust
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: hpa at zytor dot com
  Target Milestone: ---

__attribute__((interrupt)) on x86 has two prototypes, and picking the wrong
type "probably will cause a system crash." It turns out that this is
unavoidable on i386, but on x86-64 we can do better:

- On x86-64, an exception/interrupt carries an error code if and only if the
stack is 16-byte aligned (specifically, RSP[3] = 0) on exception entry.

The proper stack pointer for using with IRET is therefore always given by:

RSP |= 8

... and the error code, if present, will be located at offset -8 from this
address.

[Bug target/113321] x86-64: Make __attribute__((interrupt)) more robust

[hjl.tools at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113321

--- Comment #1 from H.J. Lu <hjl.tools at gmail dot com> ---
(In reply to H. Peter Anvin from comment #0)
> __attribute__((interrupt)) on x86 has two prototypes, and picking the wrong
> type "probably will cause a system crash." It turns out that this is
> unavoidable on i386, but on x86-64 we can do better:
> 
> - On x86-64, an exception/interrupt carries an error code if and only if the
> stack is 16-byte aligned (specifically, RSP[3] = 0) on exception entry.
> 
> The proper stack pointer for using with IRET is therefore always given by:
> 
> RSP |= 8
> 
> ... and the error code, if present, will be located at offset -8 from this
> address.

GCC always generates IRET for __attribute__((interrupt)).  If there are 2
arguments (one of them is the error code), it is an exception handler.
Otherwise, it is an interrupt handler.

[Bug target/113321] x86-64: Make __attribute__((interrupt)) more robust

[hpa at zytor dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113321

--- Comment #2 from H. Peter Anvin <hpa at zytor dot com> ---
Right. The only thing I'm suggesting is that for the cost of one extra
instruction we can make it robust against the programmer picking the wrong
type, or wanting to use the same handler.

It isn't a necessary thing by any means.