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

[Arnd Bergmann <arnd@arndb.de>]

On Thursday 14 October 2010 22:59:04 Alex Williamson wrote:
> 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.

I'm talking about the alignment of the structure, not the members
within the structure. The data structure should be compatible, but
not accesses to it.

> AFAIK, e820 is x86-only, so we don't need to worry about breaking anyone
> else.

You can use qemu to emulate an x86 pc on anything...

> 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.

Right, the data gets passed as bytes, so it hardly matters in the end.
Still the e820_add_entry assigns data to the struct members, which
it either does using byte accesses and shifts or a multiple 32 bit
assignment. Just because using a one byte alignment technically
results in correct output doesn't make it the right solution.

I don't care about the few cycles of execution time or the few bytes
you waste in this particular case, but you are setting a wrong example
by using smaller alignment than necessary.

	Arnd