On Fri, May 15, 2020 at 5:10 PM Thomas Gleixner <[email protected]> wrote: > > > Device interrupts which go through do_IRQ() or the spurious interrupt > handler have their separate entry code on 64 bit for no good reason. > > Both 32 and 64 bit transport the vector number through ORIG_[RE]AX in > pt_regs. Further the vector number is forced to fit into an u8 and is > complemented and offset by 0x80 so it's in the signed character > range. Otherwise GAS would expand the pushq to a 5 byte instruction for any > vector > 0x7F. > > Treat the vector number like an error code and hand it to the C function as > argument. This allows to get rid of the extra entry code in a later step. > > Simplify the error code push magic by implementing the pushq imm8 via a > '.byte 0x6a, vector' sequence so GAS is not able to screw it up. As the > pushq imm8 is sign extending the resulting error code needs to be truncated > to 8 bits in C code.
Acked-by: Andy Lutomirski <[email protected]> although you may be giving me more credit than deserved :) + .align 8 > +SYM_CODE_START(irq_entries_start) > + vector=FIRST_EXTERNAL_VECTOR > + .rept (FIRST_SYSTEM_VECTOR - FIRST_EXTERNAL_VECTOR) > + UNWIND_HINT_IRET_REGS > + .byte 0x6a, vector > + jmp common_interrupt > + .align 8 > + vector=vector+1 > + .endr > +SYM_CODE_END(irq_entries_start) Having battled code like this in the past (for early exceptions), I prefer the variant like: pos = .; .rept blah blah blah .byte whatever jmp whatever . = pos + 8; vector = vector + 1 .endr or maybe: .rept blah blah blah .byte whatever jmp whatever; . = irq_entries_start + 8 * vector; vector = vector + 1 .endr The reason is that these variants will fail to assemble if something goes wrong and the code expands to more than 8 bytes, whereas using .align will cause gas to happily emit 16 bytes and result in hard-to-debug mayhem.
