x86/ioemul: Account for ioemul_handle_quirk() in stub length check

The opcode potentially written into ctxt->io_emul_stub[] in the case
that ioemul_handle_quirk() is overriding the default logic isnt
accounted for in the build-time check that the stubs are large enough.

Introduce IOEMUL_QUIRK_STUB_BYTES and use for both the main and quirk
stub cases.  As a slim optimisation, avoid writing out the default stub
when we know we are going to overwrite it.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

xen/arch/x86/ioport_emulate.c

@@ -35,6 +35,8 @@ static void ioemul_handle_proliant_quirk(
     io_emul_stub[8] = 0x9d;
     /*    ret */
     io_emul_stub[9] = 0xc3;
+
+    BUILD_BUG_ON(IOEMUL_QUIRK_STUB_BYTES < 10);
 }
 
 static int __init proliant_quirk(struct dmi_system_id *d)

xen/arch/x86/pv/emul-priv-op.c

@@ -89,19 +89,24 @@ static io_emul_stub_t *io_emul_stub_setup(struct priv_op_ctxt *ctxt, u8 opcode,
     /* callq *%rcx */
     ctxt->io_emul_stub[10] = 0xff;
     ctxt->io_emul_stub[11] = 0xd1;
-    /* data16 or nop */
-    ctxt->io_emul_stub[12] = (bytes != 2) ? 0x90 : 0x66;
-    /* <io-access opcode> */
-    ctxt->io_emul_stub[13] = opcode;
-    /* imm8 or nop */
-    ctxt->io_emul_stub[14] = !(opcode & 8) ? port : 0x90;
-    /* ret (jumps to guest_to_host_gpr_switch) */
-    ctxt->io_emul_stub[15] = 0xc3;
-    BUILD_BUG_ON(STUB_BUF_SIZE / 2 < 16);
 
-    if ( ioemul_handle_quirk )
+    if ( likely(!ioemul_handle_quirk) )
+    {
+        /* data16 or nop */
+        ctxt->io_emul_stub[12] = (bytes != 2) ? 0x90 : 0x66;
+        /* <io-access opcode> */
+        ctxt->io_emul_stub[13] = opcode;
+        /* imm8 or nop */
+        ctxt->io_emul_stub[14] = !(opcode & 8) ? port : 0x90;
+        /* ret (jumps to guest_to_host_gpr_switch) */
+        ctxt->io_emul_stub[15] = 0xc3;
+    }
+    else
         ioemul_handle_quirk(opcode, &ctxt->io_emul_stub[12], ctxt->ctxt.regs);
 
+    BUILD_BUG_ON(STUB_BUF_SIZE / 2 < MAX(16, /* Regular stubs */
+                                         12 + IOEMUL_QUIRK_STUB_BYTES));
+
     /* Handy function-typed pointer to the stub. */
     return (void *)(this_cpu(stubs.addr) + STUB_BUF_SIZE / 2);
 }

xen/include/asm-x86/io.h

@@ -51,6 +51,7 @@ __OUT(l,,int)
 extern void (*pv_post_outb_hook)(unsigned int port, u8 value);
 
 /* Function pointer used to handle platform specific I/O port emulation. */
+#define IOEMUL_QUIRK_STUB_BYTES 10
 extern void (*ioemul_handle_quirk)(
     u8 opcode, char *io_emul_stub, struct cpu_user_regs *regs);