[Qemu-devel] [PATCH 5/5] pseries: Allow kernel's early debug output to work

[David Gibson]

The PAPR specification defines a virtual TTY/console interface for guest
OSes to use via the H_PUT_TERM_CHAR and H_GET_TERM_CHAR hypercalls.  There
can be multiple virtual ttys, so these take a "termno" parameter.  This
encodes which vty to use as the 'reg' property on the device tree node
associated with that vty.

However, with the early debug options enabled, the Linux kernel will
attempt debugging output through the vty very early, before it has read
the device tree.  In this case it always uses a termno of 0.  This works
on the existing PowerVM hypervisor, so we assume there must be a hack /
feature in there which interprets termno==0 to mean the default primary
console.

To help with debugging kernels, including existing distribution kernels,
this patch implements a similar feature / hack in qemu.  If termno==0
is supplied to H_{GET,PUT}_TERM_CHAR, they use the first available vty
device instead.

We need to be careful in the case that the user has manually created
an spapr-vty at address 0. So first we search for the specified reg and
only if that doesn't match do we fall back.

Signed-off-by: Michael Ellerman <address@hidden>
Signed-off-by: David Gibson <address@hidden>
---
 hw/spapr_vty.c |   32 ++++++++++++++++++++++++++++++--
 1 files changed, 30 insertions(+), 2 deletions(-)

diff --git a/hw/spapr_vty.c b/hw/spapr_vty.c
index 8150395..f23cc36 100644
--- a/hw/spapr_vty.c
+++ b/hw/spapr_vty.c
@@ -69,6 +69,9 @@ static int spapr_vty_init(VIOsPAPRDevice *sdev)
     return 0;
 }
 
+/* Forward declaration */
+static VIOsPAPRDevice *vty_lookup(sPAPREnvironment *spapr, target_ulong reg);
+
 static target_ulong h_put_term_char(CPUState *env, sPAPREnvironment *spapr,
                                     target_ulong opcode, target_ulong *args)
 {
@@ -76,9 +79,10 @@ static target_ulong h_put_term_char(CPUState *env, 
sPAPREnvironment *spapr,
     target_ulong len = args[1];
     target_ulong char0_7 = args[2];
     target_ulong char8_15 = args[3];
-    VIOsPAPRDevice *sdev = spapr_vio_find_by_reg(spapr->vio_bus, reg);
+    VIOsPAPRDevice *sdev;
     uint8_t buf[16];
 
+    sdev = vty_lookup(spapr, reg);
     if (!sdev) {
         return H_PARAMETER;
     }
@@ -102,9 +106,10 @@ static target_ulong h_get_term_char(CPUState *env, 
sPAPREnvironment *spapr,
     target_ulong *len = args + 0;
     target_ulong *char0_7 = args + 1;
     target_ulong *char8_15 = args + 2;
-    VIOsPAPRDevice *sdev = spapr_vio_find_by_reg(spapr->vio_bus, reg);
+    VIOsPAPRDevice *sdev;
     uint8_t buf[16];
 
+    sdev = vty_lookup(spapr, reg);
     if (!sdev) {
         return H_PARAMETER;
     }
@@ -151,6 +156,29 @@ static VIOsPAPRDeviceInfo spapr_vty = {
     },
 };
 
+static VIOsPAPRDevice *vty_lookup(sPAPREnvironment *spapr, target_ulong reg)
+{
+    VIOsPAPRDevice *sdev;
+
+    sdev = spapr_vio_find_by_reg(spapr->vio_bus, reg);
+    if (!sdev && reg == 0) {
+        DeviceState *qdev;
+
+        /* Hack for kernel early debug, which always specifies reg==0.
+         * We search all VIO devices, and grab the first available vty
+         * device.  This attempts to mimic existing PowerVM behaviour
+         * (early debug does work there, despite having no vty with
+         * reg==0. */
+        QTAILQ_FOREACH(qdev, &spapr->vio_bus->bus.children, sibling) {
+            if (qdev->info == &spapr_vty.qdev) {
+                return DO_UPCAST(VIOsPAPRDevice, qdev, qdev);
+            }
+        }
+    }
+
+    return sdev;
+}
+
 static void spapr_vty_register(void)
 {
     spapr_vio_bus_register_withprop(&spapr_vty);
-- 
1.7.7.1