From mboxrd@z Thu Jan  1 00:00:00 1970
From: Hollis Blanchard <hollisb-r/Jw6+rmf7HQT0dZR+AlfA@public.gmane.org>
Subject: Re: [kvm-ppc-devel] RFC: MMIO endianness flag
Date: Mon, 14 Jan 2008 21:54:18 -0600
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To: "Xu, Anthony" <anthony.xu-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>
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On Tue, 2008-01-15 at 10:43 +0800, Xu, Anthony wrote:
> > Here is the concrete example:
> >       * guest writes to MMIO
> >       * KVM passes MMIO information (physical address, number of
> >         bytes, value) to qemu
> 		The value is saved in memory, is it bigendian or
> littleendian?

The value in memory is copied from the value in the register when the
guest was executing, so its format is probably dependent on the state of
a control register.

> >       * Qemu knows from the address that this access is for a
> >         passthough device, a special case the administrator has
> >       pre-configured * Qemu does mmap(/dev/mem), and writes "length"
> 
> 		When qemu writes value, Can qemu know what
> mode(bigendian/littleendian it is running)?
> 	Qemu can run on bigendian in IA64.

/usr/include/endian.h will #define __BYTE_ORDER as either
__LITTLE_ENDIAN or __BIG_ENDIAN. I have no idea if this is defined in a
standard or is glibc-specific.

You could also test at runtime with a construct like:
        union {
                int i;
                char c[4];
        } u;
        
        u.i = 1;
        if (u.c[0] == 1) {
                ...
        } else {
                ...
        }

-- 
Hollis Blanchard
IBM Linux Technology Center


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