Re: [Qemu-devel] [PATCH] docs: memory.txt document the endian field

[Andreas Färber]

Am 12.02.2012 16:06, schrieb Michael S. Tsirkin:
> So I think the following is right?
> 
> 
> commit 02aa79aac9bec1c8c17d1b7b5405b59b649dfdb9
> Author: Michael S. Tsirkin <address@hidden>
> Date:   Wed Feb 8 17:16:35 2012 +0200
> 
>     docs: memory.txt document the endian field
>     
>     This is an attempt to document the endian
>     field in memory API. As this is a confusing topic,
>     add some examples.
>     
>     Signed-off-by: Michael S. Tsirkin <address@hidden>
> 
> diff --git a/docs/memory.txt b/docs/memory.txt
> index 5bbee8e..9132c86 100644
> --- a/docs/memory.txt
> +++ b/docs/memory.txt
> @@ -170,3 +170,48 @@ various constraints can be supplied to control how these 
> callbacks are called:
>   - .old_portio and .old_mmio can be used to ease porting from code using
>     cpu_register_io_memory() and register_ioport().  They should not be used
>     in new code.
> +- .endianness; specifies the device endian-ness, which affects
> +   the handling of the value parameter passed from guest to write
> +   and returned to guest from read callbacks, as follows:
> +        void write(void *opaque, target_phys_addr_t addr,
> +                   uint64_t value, unsigned size)
> +        uint64_t read(void *opaque, target_phys_addr_t addr,
> +                       unsigned size)
> +   value is always passed in the natural host format,
> +   low size bytes in value are set, the rest are zero padded
> +   on input and ignored on output.

Looks good so far.

> +   Legal values for endian-ness are:
> +   DEVICE_NATIVE_ENDIAN - The value is left in the format used by guest.
> +       Note that although this is typically a fixed format as
> +       guest drivers take care of endian conversions,

> +       if host endian-ness does not match the device this will
> +       result in "mixed endian" since the data is always
> +       stored in low bits of value.

Why "mixed" endian? The host always uses host endianness, and with
"native" we use the (nominal) endianness of the target.

Note that the endianness of the guest might be different from the
target's if the CPU is bi-endian.

> +
> +       To handle this data, on write, you typically need to first
> +       convert to the appropriate type, removing the
> +       padding. On read, handle the data in the appropriate
> +       type and then convert to uint64_t, padding with leading zeroes.

That applies to all three endiannesses, doesn't it?

Andreas

> +
> +   DEVICE_LITTLE_ENDIAN - The value is assumed to be
> +       endian, and is converted to host endian.
> +   DEVICE_BIG_ENDIAN - The value is assumed to be
> +        big endian, and is converted to host endian.
> +
> +    As an example, consider a little endian guest writing a 32 bit
> +    value 0x12345678 into an MMIO register, on a big endian host.
> +    The value passed to the write callback is documented below:
> +
> +   DEVICE_NATIVE_ENDIAN - value = 0x0000000087654321
> +        Explanation: write callback will get the high bits
> +        in value set to 0, and low bits set to data left
> +        as is, that is in little endian format.
> +   DEVICE_LITTLE_ENDIAN - value = 0x0000000012345678
> +        Explanation: the write callback will get the high bits
> +        in value set to 0, and low bits set to data in big endian
> +        format.
> +   DEVICE_BIG_ENDIAN - value = 0x0000000087654321
> +        Explanation: the write callback will get the high bits
> +        in value set to 0, and low bits set to data in little endian
> +        format.
> +

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