[Qemu-devel] [PATCH v4 7/7] fw_cfg: Define a static signature to be returned on DMA port reads

["Marc Marí" <markmb@redhat.com>]

From: Kevin O'Connor <kevin@koconnor.net>

Return a static signature ("QEMU CFG") if the guest does a read to the
DMA address io register.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>
---
 docs/specs/fw_cfg.txt |  4 ++++
 hw/nvram/fw_cfg.c     | 13 +++++++++++--
 2 files changed, 15 insertions(+), 2 deletions(-)

diff --git a/docs/specs/fw_cfg.txt b/docs/specs/fw_cfg.txt
index 2d6b2da..249b99e 100644
--- a/docs/specs/fw_cfg.txt
+++ b/docs/specs/fw_cfg.txt
@@ -93,6 +93,10 @@ by selecting the "signature" item using key 0x0000 (FW_CFG_SIGNATURE),
 and reading four bytes from the data register. If the fw_cfg device is
 present, the four bytes read will contain the characters "QEMU".
 
+Additionaly, if the DMA interface is available then a read to the DMA
+Address will return 0x51454d5520434647 ("QEMU CFG" in big-endian
+format).
+
 === Revision / feature bitmap (Key 0x0001, FW_CFG_ID) ===
 
 A 32-bit little-endian unsigned int, this item is used to check for enabled
diff --git a/hw/nvram/fw_cfg.c b/hw/nvram/fw_cfg.c
index 59933b3..c6dcce4 100644
--- a/hw/nvram/fw_cfg.c
+++ b/hw/nvram/fw_cfg.c
@@ -53,6 +53,8 @@
 #define FW_CFG_DMA_CTL_SKIP    0x04
 #define FW_CFG_DMA_CTL_SELECT  0x08
 
+#define FW_CFG_DMA_SIGNATURE 0x51454d5520434647 /* "QEMU CFG" */
+
 typedef struct FWCfgEntry {
     uint32_t len;
     uint8_t *data;
@@ -393,6 +395,12 @@ static void fw_cfg_dma_transfer(FWCfgState *s)
     trace_fw_cfg_read(s, 0);
 }
 
+static uint64_t fw_cfg_dma_mem_read(void *opaque, hwaddr addr,
+                                    unsigned size)
+{
+    return FW_CFG_DMA_SIGNATURE >> ((8 - addr - size) * 8);
+}
+
 static void fw_cfg_dma_mem_write(void *opaque, hwaddr addr,
                                  uint64_t value, unsigned size)
 {
@@ -416,8 +424,8 @@ static void fw_cfg_dma_mem_write(void *opaque, hwaddr addr,
 static bool fw_cfg_dma_mem_valid(void *opaque, hwaddr addr,
                                   unsigned size, bool is_write)
 {
-    return is_write && ((size == 4 && (addr == 0 || addr == 4)) ||
-                        (size == 8 && addr == 0));
+    return !is_write || ((size == 4 && (addr == 0 || addr == 4)) ||
+                         (size == 8 && addr == 0));
 }
 
 static bool fw_cfg_data_mem_valid(void *opaque, hwaddr addr,
@@ -488,6 +496,7 @@ static const MemoryRegionOps fw_cfg_comb_mem_ops = {
 };
 
 static const MemoryRegionOps fw_cfg_dma_mem_ops = {
+    .read = fw_cfg_dma_mem_read,
     .write = fw_cfg_dma_mem_write,
     .endianness = DEVICE_BIG_ENDIAN,
     .valid.accepts = fw_cfg_dma_mem_valid,
-- 
2.4.3