Re: [Qemu-devel] [PATCHv2] fw_cfg: Define a static signature to be returned on DMA port reads

[Laszlo Ersek <lersek@redhat.com>]

On 10/06/15 01:51, Kevin O'Connor wrote:
> 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>
> ---
> 
> Marc, if you decide to respin your fw_cfg series, I've updated the dma
> signature patch.  This addresses the comments from Stefan, and I hope
> it addresses the comments from Laszlo.

Thank you -- I didn't know about extract64().

The patch looks good to me, but I think the QEMU coding style requries
/* ... */ comments, and forbids //.

... "scripts/checkpatch.pl" has the following snippet:

# no C99 // comments
                if ($line =~ m{//}) {
                        ERROR("do not use C99 // comments\n" . $herecurr);
                }
...

Thanks!
Laszlo

> 
> BTW, if you wanted to, it's possible to use deposit64 in
> fw_cfg_dma_mem_write() to support all possible (validly aligned) write
> sizes.  Then fw_cfg_dma_mem_valid() shouldn't be needed.  Something
> like:
> 
> static void fw_cfg_dma_mem_write(void *opaque, hwaddr addr,
>                                  uint64_t value, unsigned size)
> {
>     FWCfgState *s = opaque;
>     s->dma_addr = deposit64(s->dma_addr, (8 - addr - size)*8, size*8, value);
>     if (addr + size >= 8) {
>         fw_cfg_dma_transfer(s);
>     }
> }
> 
> ---
>  docs/specs/fw_cfg.txt |  3 +++
>  hw/nvram/fw_cfg.c     | 14 ++++++++++++--
>  2 files changed, 15 insertions(+), 2 deletions(-)
> 
> diff --git a/docs/specs/fw_cfg.txt b/docs/specs/fw_cfg.txt
> index 2d6b2da..cbdce7d 100644
> --- a/docs/specs/fw_cfg.txt
> +++ b/docs/specs/fw_cfg.txt
> @@ -93,6 +93,9 @@ 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".
>  
> +If the DMA interface is available, then reading the DMA Address
> +Register returns 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..cf5c5c4 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 +395,13 @@ 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 a signature value (and handle various read sizes)
> +    return extract64(FW_CFG_DMA_SIGNATURE, (8 - addr - size) * 8, size*8);
> +}
> +
>  static void fw_cfg_dma_mem_write(void *opaque, hwaddr addr,
>                                   uint64_t value, unsigned size)
>  {
> @@ -416,8 +425,8 @@ 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 +497,7 @@ 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,
>