Re: [Qemu-devel] [PATCH 01/15] exec: introduce endianness swapped mmio

[Andreas Färber]

Am 25.11.2010 um 08:35 schrieb Alexander Graf:

> The way we're currently modeling mmio is too simplified. We assume  
> that
> every device has the same endianness as the target CPU. In reality,
> most devices are little endian (all PCI and ISA ones I'm aware of).  
> Some
> are big endian (special system devices) and a very little fraction is
> target native endian (fw_cfg).
>
> So instead of assuming every device to be native endianness, let's  
> move
> to a model where the device tells us which endianness it's in.
>
> That way we can compile the devices only once and get rid of all the  
> ugly
> swap will be done by the underlying layer.
>
> For the same of readability, this patch only introduces the helper  
> framework
> but doesn't allow the registering code to set its endianness yet.
>
> Signed-off-by: Alexander Graf <address@hidden>

> ---
> cpu-common.h |    4 ++
> exec.c       |  122 +++++++++++++++++++++++++++++++++++++++++++++++++ 
> +++++++++
> 2 files changed, 126 insertions(+), 0 deletions(-)
>
> diff --git a/cpu-common.h b/cpu-common.h
> index a543b5d..839b236 100644
> --- a/cpu-common.h
> +++ b/cpu-common.h
> @@ -20,6 +20,10 @@
>
> #if !defined(CONFIG_USER_ONLY)
>
> +#define DEVICE_NATIVE_ENDIAN  0
> +#define DEVICE_BIG_ENDIAN     1
> +#define DEVICE_LITTLE_ENDIAN  2

I believe some people around here voiced a preference for enums, to  
aid with gdb debugging?

> diff --git a/exec.c b/exec.c
> index db9ff55..f54a360 100644
> --- a/exec.c
> +++ b/exec.c

> @@ -3370,6 +3474,22 @@ static int cpu_register_io_memory_fixed(int  
> io_index,
>     }
>     io_mem_opaque[io_index] = opaque;
>
> +    switch (endian) {
> +    case DEVICE_BIG_ENDIAN:
> +#ifndef TARGET_WORDS_BIGENDIAN
> +        swapendian_init(io_index);
> +#endif
> +        break;

So basically, you just moved the #ifdefs to another place. :)  
Shouldn't this be dependent on the CPU state and determined at  
runtime? Thinking of MSR LE bit on ppc. I guess QEMU doesn't support  
bi-endian ppc today, as does the 970, but it would be nice to keep it  
in mind.

Andreas