Re: [Qemu-devel] Re: [PATCH] pc: e820 qemu_cfg tables need to be packed

[Alex Williamson <alex.williamson@redhat.com>]

On Thu, 2010-10-14 at 22:20 +0200, Arnd Bergmann wrote:
> On Thursday 14 October 2010 21:58:08 Alex Williamson wrote:
> > If it works anywhere (I assume it works on 32bit), then it's only
> > because it happened to get the alignment right.  This just makes 64bit
> > hosts get it right too.  I don't see any compatibility issues,
> > non-packed + 64bit = broken.  Thanks,
> 
> I would actually assume that only x86-32 hosts got it right, because
> all 32 bit hosts I've seen other than x86 also define 8 byte alignment
> for uint64_t.
> 
> You might however consider making it 
> 
> __attribute((__packed__, __aligned__(4)))
> 
> instead of just packed, because otherwise you make the alignment one byte,
> which is not only different from what it used to be on x86-32 but also
> will cause inefficient compiler outpout on platforms that don't have unaligned
> word accesses in hardware.

The structs in question only contain 4 & 8 byte elements, so there
shouldn't be any change on x86-32 using one-byte aligned packing.
AFAIK, e820 is x86-only, so we don't need to worry about breaking anyone
else.  Performance isn't much of a consideration for this type of
interface since it's only used pre-boot.  In fact, the channel between
qemu and the bios is only one byte wide, so wider alignment can cost
extra emulated I/O accesses.  Thanks,

Alex