On 7 August 2014 16:06, Richard W.M. Jones <rjo...@redhat.com> wrote: > On Thu, Aug 07, 2014 at 02:53:33PM +0100, Peter Maydell wrote: >> I think the biggest difficulty is not the code to actually >> do the switch but figuring out what the command line >> UI to request "start in AArch32" ought to be and how that >> then gets plumbed into the code to do the actual work. > > Out of interest, how do you do this on real hardware?
Depends what level you're thinking at. At the absolute lowest level, the Cortex-A57 for instance has an input signal that's sampled on reset that indicates whether it starts in AArch32 or AArch64. (In QEMU terms that would be a CPU QOM property.) I'd expect that most boards in practice would just hardwire that, though, since the first thing that runs is going to be board-specific bootrom code and having that be multi-width would be pointless when you can just have the bootrom start EL2 in AArch32 if it really needs to. I don't know what the typical boot loader does. For UEFI the files it boots are PE/COFF so in theory it could look at the header (but I'm told that at the moment 64-bit UEFI simply assumes 64-bit kernels). I think u-boot also currently assumes 64-bit kernels if it's a 64-bit u-boot. Basically virtual machines are about the only place where you'd actually want to run a 32 bit kernel if your hardware supports 64 bit. So the only precedent we have here is kvmtool, which currently requires you to pass a command line argument if you want your 64-bit guest CPU to start in AArch32. thanks -- PMM
