lguest: don't look in console features to find emerg_wr.

The 1.0 spec clearly states that you must set the ACKNOWLEDGE and
DRIVER status bits before accessing the feature bits.  This is a
problem for the early console code, which doesn't really want to
acknowledge the device (the spec specifically excepts writing to the
console's emerg_wr from the usual ordering constrains).

Instead, we check that the *size* of the device configuration is
sufficient to hold emerg_wr: at worst (if the device doesn't support
the VIRTIO_CONSOLE_F_EMERG_WRITE feature), it will ignore the
writes.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

1 files changed, 24 insertions, 33 deletions

diff --git a/arch/x86/lguest/boot.c b/arch/x86/lguest/boot.c
index 531b844cb48d..ac4453d8520e 100644
--- a/arch/x86/lguest/boot.c
+++ b/arch/x86/lguest/boot.c
@@ -1222,15 +1222,13 @@ static void set_cfg_window(u32 cfg_offset, u32 off)
                          off);
 }
 
-static u32 read_bar_via_cfg(u32 cfg_offset, u32 off)
-{
-        set_cfg_window(cfg_offset, off);
-        return read_pci_config(0, 1, 0,
-                               cfg_offset + sizeof(struct virtio_pci_cap));
-}
-
 static void write_bar_via_cfg(u32 cfg_offset, u32 off, u32 val)
 {
+        /*
+         * We could set this up once, then leave it; nothing else in the *
+         * kernel should touch these registers.  But if it went wrong, that
+         * would be a horrible bug to find.
+         */
         set_cfg_window(cfg_offset, off);
         write_pci_config(0, 1, 0,
                          cfg_offset + sizeof(struct virtio_pci_cap), val);
@@ -1239,8 +1237,9 @@ static void write_bar_via_cfg(u32 cfg_offset, u32 off, u32 val)
 static void probe_pci_console(void)
 {
         u8 cap, common_cap = 0, device_cap = 0;
-        /* Offsets within BAR0 */
-        u32 common_offset, device_offset;
+        /* Offset within BAR0 */
+        u32 device_offset;
+        u32 device_len;
 
         /* Avoid recursive printk into here. */
         console_cfg_offset = -1;
@@ -1263,7 +1262,7 @@ static void probe_pci_console(void)
                 u8 vndr = read_pci_config_byte(0, 1, 0, cap);
                 if (vndr == PCI_CAP_ID_VNDR) {
                         u8 type, bar;
-                        u32 offset;
+                        u32 offset, length;
 
                         type = read_pci_config_byte(0, 1, 0,
                             cap + offsetof(struct virtio_pci_cap, cfg_type));
@@ -1271,18 +1270,15 @@ static void probe_pci_console(void)
                             cap + offsetof(struct virtio_pci_cap, bar));
                         offset = read_pci_config(0, 1, 0,
                             cap + offsetof(struct virtio_pci_cap, offset));
+                        length = read_pci_config(0, 1, 0,
+                            cap + offsetof(struct virtio_pci_cap, length));
 
                         switch (type) {
-                        case VIRTIO_PCI_CAP_COMMON_CFG:
-                                if (bar == 0) {
-                                        common_cap = cap;
-                                        common_offset = offset;
-                                }
-                                break;
                         case VIRTIO_PCI_CAP_DEVICE_CFG:
                                 if (bar == 0) {
                                         device_cap = cap;
                                         device_offset = offset;
+                                        device_len = length;
                                 }
                                 break;
                         case VIRTIO_PCI_CAP_PCI_CFG:
@@ -1292,32 +1288,27 @@ static void probe_pci_console(void)
                 }
                 cap = read_pci_config_byte(0, 1, 0, cap + PCI_CAP_LIST_NEXT);
         }
-        if (!common_cap || !device_cap || !console_access_cap) {
+        if (!device_cap || !console_access_cap) {
                 printk(KERN_ERR "lguest: No caps (%u/%u/%u) in console!\n",
                        common_cap, device_cap, console_access_cap);
                 return;
         }
 
-
-#define write_common_config(reg, val)                                   \
-        write_bar_via_cfg(console_access_cap,                           \
-                          common_offset+offsetof(struct virtio_pci_common_cfg,reg),\
-                          val)
-
-#define read_common_config(reg)                                         \
-        read_bar_via_cfg(console_access_cap,                            \
-                         common_offset+offsetof(struct virtio_pci_common_cfg,reg))
-
-        /* Check features: they must offer EMERG_WRITE */
-        write_common_config(device_feature_select, 0);
-
-        if (!(read_common_config(device_feature)
-              & (1 << VIRTIO_CONSOLE_F_EMERG_WRITE))) {
-                printk(KERN_ERR "lguest: console missing EMERG_WRITE\n");
+        /*
+         * Note that we can't check features, until we've set the DRIVER
+         * status bit.  We don't want to do that until we have a real driver,
+         * so we just check that the device-specific config has room for
+         * emerg_wr.  If it doesn't support VIRTIO_CONSOLE_F_EMERG_WRITE
+         * it should ignore the access.
+         */
+        if (device_len < (offsetof(struct virtio_console_config, emerg_wr)
+                          + sizeof(u32))) {
+                printk(KERN_ERR "lguest: console missing emerg_wr field\n");
                 return;
         }
 
         console_cfg_offset = device_offset;
+        printk(KERN_INFO "lguest: Console via virtio-pci emerg_wr\n");
 }
 
 /*