Re: [Qemu-devel] [PATCH v5 5/6] fw_cfg: add generic non-DMA read method

[Laszlo Ersek <lersek@redhat.com>]

On 11/05/15 15:32, Gabriel L. Somlo wrote:
> Introduce fw_cfg_data_read(), a generic read method which works
> on all access widths (1 through 8 bytes, inclusive), and can be
> used during both IOPort and MMIO read accesses.
> 
> To maintain legibility, only fw_cfg_data_mem_read() (the MMIO
> data read method) is replaced by this patch. The new method
> essentially unwinds the fw_cfg_data_mem_read() + fw_cfg_read()
> combo, but without unnecessarily repeating all the validity
> checks performed by the latter on each byte being read.
> 
> This patch also modifies the trace_fw_cfg_read prototype to
> accept a 64-bit value argument, allowing it to work properly
> with the new read method, but also remain backward compatible
> with existing call sites.
> 
> Cc: Laszlo Ersek <lersek@redhat.com>
> Cc: Gerd Hoffmann <kraxel@redhat.com>
> Cc: Marc Marí <markmb@redhat.com>
> Signed-off-by: Gabriel Somlo <somlo@cmu.edu>
> ---
> 
> I think in this case I'll stick with a single assert for the
> whole bounds check. In the long run, "easy to read" might be
> just a bit more helpful than "in the VERY unlikely event of
> actually triggering this, it will also tell us which side of
> the interval we fell out on"...
> 
> Thanks again for all the advice and feedback!
>   --Gabriel
> 
>  hw/nvram/fw_cfg.c | 45 +++++++++++++++++++++++++++++++--------------
>  trace-events      |  2 +-
>  2 files changed, 32 insertions(+), 15 deletions(-)
> 
> diff --git a/hw/nvram/fw_cfg.c b/hw/nvram/fw_cfg.c
> index 046fa74..8f566f6 100644
> --- a/hw/nvram/fw_cfg.c
> +++ b/hw/nvram/fw_cfg.c
> @@ -274,6 +274,36 @@ static int fw_cfg_select(FWCfgState *s, uint16_t key)
>      return ret;
>  }
>  
> +static uint64_t fw_cfg_data_read(void *opaque, hwaddr addr, unsigned size)
> +{
> +    FWCfgState *s = opaque;
> +    int arch = !!(s->cur_entry & FW_CFG_ARCH_LOCAL);
> +    FWCfgEntry *e = (s->cur_entry == FW_CFG_INVALID) ? NULL :
> +                    &s->entries[arch][s->cur_entry & FW_CFG_ENTRY_MASK];
> +    uint64_t value = 0;
> +
> +    assert(size > 0 && size <= sizeof(value));
> +    if (s->cur_entry != FW_CFG_INVALID && e->data && s->cur_offset < e->len) {
> +        /* The least significant 'size' bytes of the return value are
> +         * expected to contain a string preserving portion of the item
> +         * data, padded with zeros on the right in case we run out early.
> +         * In technical terms, we're composing the host-endian representation
> +         * of the big endian interpretation of the fw_cfg string.
> +         */
> +        do {
> +            value = (value << 8) | e->data[s->cur_offset++];
> +        } while (--size && s->cur_offset < e->len);
> +        /* If size is still not zero, we *did* run out early, so continue
> +         * left-shifting, to add the appropriate number of padding zeros
> +         * on the right.
> +         */
> +        value <<= 8 * size;
> +    }
> +
> +    trace_fw_cfg_read(s, value);
> +    return value;
> +}
> +
>  static uint8_t fw_cfg_read(FWCfgState *s)
>  {
>      int arch = !!(s->cur_entry & FW_CFG_ARCH_LOCAL);
> @@ -291,19 +321,6 @@ static uint8_t fw_cfg_read(FWCfgState *s)
>      return ret;
>  }
>  
> -static uint64_t fw_cfg_data_mem_read(void *opaque, hwaddr addr,
> -                                     unsigned size)
> -{
> -    FWCfgState *s = opaque;
> -    uint64_t value = 0;
> -    unsigned i;
> -
> -    for (i = 0; i < size; ++i) {
> -        value = (value << 8) | fw_cfg_read(s);
> -    }
> -    return value;
> -}
> -
>  static void fw_cfg_data_mem_write(void *opaque, hwaddr addr,
>                                    uint64_t value, unsigned size)
>  {
> @@ -485,7 +502,7 @@ static const MemoryRegionOps fw_cfg_ctl_mem_ops = {
>  };
>  
>  static const MemoryRegionOps fw_cfg_data_mem_ops = {
> -    .read = fw_cfg_data_mem_read,
> +    .read = fw_cfg_data_read,
>      .write = fw_cfg_data_mem_write,
>      .endianness = DEVICE_BIG_ENDIAN,
>      .valid = {
> diff --git a/trace-events b/trace-events
> index 17fbddf..f2f2ce4 100644
> --- a/trace-events
> +++ b/trace-events
> @@ -196,7 +196,7 @@ ecc_diag_mem_readb(uint64_t addr, uint32_t ret) "Read diagnostic %"PRId64"= %02x
>  
>  # hw/nvram/fw_cfg.c
>  fw_cfg_select(void *s, uint16_t key, int ret) "%p key %d = %d"
> -fw_cfg_read(void *s, uint8_t ret) "%p = %d"
> +fw_cfg_read(void *s, uint64_t ret) "%p = %"PRIx64
>  fw_cfg_add_file(void *s, int index, char *name, size_t len) "%p #%d: %s (%zd bytes)"
>  
>  # hw/block/hd-geometry.c
> 

Reviewed-by: Laszlo Ersek <lersek@redhat.com>