Re: [Qemu-devel] [PATCH v3 0/5] fw_cfg DMA interface

[Kevin O'Connor <kevin@koconnor.net>]

On Fri, Sep 18, 2015 at 10:58:44AM +0200, Marc Marí wrote:
> Implement host-side of the FW CFG DMA interface both for x86 and ARM.
> 
> Based on Gerd Hoffman's initial implementation.

Thanks for working on this Marc!

Any chance you could add the patch below to the series (or merge it
into your series)?

The patch adds a signature to the DMA address IO register.  With the
current implementation, a future firmware would have to implement the
V1 fw_cfg interface just to probe for the dma interface.  It might be
useful if future firmwares (that don't care about backwards
compatibility with old versions of qemu) could probe for the dma
fw_cfg interface by just checking for a signature (and therefore not
require all the V1 code just to probe).

-Kevin


commit ae6d8df012ef9b21ae17bfb0383d116f71ba1d58
Author: Kevin O'Connor <kevin@koconnor.net>
Date:   Fri Sep 18 14:14:55 2015 -0400

    fw_cfg: Define a static signature to be returned on DMA port reads
    
    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>

diff --git a/docs/specs/fw_cfg.txt b/docs/specs/fw_cfg.txt
index d5f9ddd..5bf3f65 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_SIGNATU
RE),
 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 d11d8c5..d95075d 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,