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

[Michael S. Tsirkin]

On Sun, Feb 12, 2012 at 07:20:07PM +0100, Andreas Färber wrote:
> 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.
> > +
> 


It looks like the text is wrong anyway.
I give up for now, maybe Avi can document it
properly.


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