x86/irq: Convey vector as argument and not in ptregs

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.

Originally-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Andy Lutomirski <luto@kernel.org>
Link: https://lore.kernel.org/r/20200521202118.796915981@linutronix.de

1 files changed, 48 insertions, 0 deletions

diff --git a/arch/x86/include/asm/idtentry.h b/arch/x86/include/asm/idtentry.h
index 36e5b929389b..2fc0dc8af2a4 100644
--- a/arch/x86/include/asm/idtentry.h
+++ b/arch/x86/include/asm/idtentry.h
@@ -347,6 +347,54 @@ __visible noinstr void func(struct pt_regs *regs,			\
 #define DECLARE_IDTENTRY_XEN(vector, func)				\
 	idtentry vector asm_exc_xen##func exc_##func has_error_code=0
 
+/*
+ * ASM code to emit the common vector entry stubs where each stub is
+ * packed into 8 bytes.
+ *
+ * Note, that the 'pushq imm8' is emitted via '.byte 0x6a, vector' because
+ * GCC treats the local vector variable as unsigned int and would expand
+ * all vectors above 0x7F to a 5 byte push. The original code did an
+ * adjustment of the vector number to be in the signed byte range to avoid
+ * this. While clever it's mindboggling counterintuitive and requires the
+ * odd conversion back to a real vector number in the C entry points. Using
+ * .byte achieves the same thing and the only fixup needed in the C entry
+ * point is to mask off the bits above bit 7 because the push is sign
+ * extending.
+ */
+	.align 8
+SYM_CODE_START(irq_entries_start)
+    vector=FIRST_EXTERNAL_VECTOR
+    pos = .
+    .rept (FIRST_SYSTEM_VECTOR - FIRST_EXTERNAL_VECTOR)
+	UNWIND_HINT_IRET_REGS
+	.byte	0x6a, vector
+	jmp	common_interrupt
+	nop
+	/* Ensure that the above is 8 bytes max */
+	. = pos + 8
+    pos=pos+8
+    vector=vector+1
+    .endr
+SYM_CODE_END(irq_entries_start)
+
+#ifdef CONFIG_X86_LOCAL_APIC
+	.align 8
+SYM_CODE_START(spurious_entries_start)
+    vector=FIRST_SYSTEM_VECTOR
+    pos = .
+    .rept (NR_VECTORS - FIRST_SYSTEM_VECTOR)
+	UNWIND_HINT_IRET_REGS
+	.byte	0x6a, vector
+	jmp	common_spurious
+	nop
+	/* Ensure that the above is 8 bytes max */
+	. = pos + 8
+    pos=pos+8
+    vector=vector+1
+    .endr
+SYM_CODE_END(spurious_entries_start)
+#endif
+
 #endif /* __ASSEMBLY__ */
 
 /*