[PATCH for-4.21] x86/hvm: fix reading from 0xe9 IO port if port E9 hack is active

[Roger Pau Monne <roger.pau@citrix.com>]

Reading from the E9 port if the emergency console is active should return
0xe9 according to the documentation from Bochs:

https://bochs.sourceforge.io/doc/docbook/user/bochsrc.html

See `port_e9_hack` section description.

Fix Xen so it also returns the port address.  OSes can use it to detect
whether the emergency console is available or not.

Fixes: d1bd157fbc9b ("Big merge the HVM full-virtualisation abstractions.")
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
---
 xen/arch/x86/hvm/hvm.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/xen/arch/x86/hvm/hvm.c b/xen/arch/x86/hvm/hvm.c
index 23bd7f078a1d..0c60faa39d7b 100644
--- a/xen/arch/x86/hvm/hvm.c
+++ b/xen/arch/x86/hvm/hvm.c
@@ -567,9 +567,15 @@ static int cf_check hvm_print_line(
 
     ASSERT(bytes == 1 && port == XEN_HVM_DEBUGCONS_IOPORT);
 
-    /* Deny any input requests. */
-    if ( dir != IOREQ_WRITE )
-        return X86EMUL_UNHANDLEABLE;
+    if ( dir == IOREQ_READ )
+    {
+        /*
+         * According to Bochs documentation, reading from the E9 port should
+         * return 0xE9 if the "port E9 hack" is implemented.
+         */
+        *val = XEN_HVM_DEBUGCONS_IOPORT;
+        return X86EMUL_OKAY;
+    }
 
     if ( !is_console_printable(c) )
         return X86EMUL_OKAY;
-- 
2.51.0