On Wed, 2017-06-21 at 13:45 -0700, Anatol Pomozov wrote: > Hi Adam, thank you for your reply. > > On Mon, Jun 19, 2017 at 2:08 PM, Adam Lackorzynski > <a...@os.inf.tu-dresden.de> wrote: > > Hi, > > > > On Tue Jun 13, 2017 at 17:05:41 -0700, Anatol Pomozov wrote: > > > Do these arguments sound reasonable to apply the patch? > > > > I'm not really convinced. > > > > > On Thu, Jun 8, 2017 at 2:07 PM, Anatol Pomozov <anatol.pomo...@gmail.com> > > > wrote: > > > > +reply-all > > > > > > > > On Thu, Jun 8, 2017 at 1:41 PM, Adam Lackorzynski > > > > <a...@os.inf.tu-dresden.de> wrote: > > > > > > > > > > On Tue Jun 06, 2017 at 21:41:48 -0700, Anatol Pomozov wrote: > > > > > > It is possible to create a 64 bit elf image that has valid > > > > > > multiboot header. > > > > > > qemu should be able to boot such images. > > > > > > > > > > But this 64bit image actually starts with 32bit code, right? > > > > > > > > Correct. The very first part of the startup code has to be 32bit. > > > > After it sets "long mode" it can use 64bit instructions. To make sure > > > > that the preamble has only 32bit instructions one have to use asm > > > > directive such as ".code32". > > > > > > > > Here is an example from LitleKernel sturtup code: > > > > > > > > https://github.com/littlekernel/lk/blob/master/arch/x86/64/start.S#L50 > > > > > > > > .code32 tells assembler to treat following text as 32 bit code. And > > > > later when it jumps into "long mode" > > > > > > > > https://github.com/littlekernel/lk/blob/master/arch/x86/64/start.S#L214 > > > > one can use 64bit code. > > > > > > > > > So it's a 32bit program and the check verifies that this is the case. > > > > > > > > While preamble have to contain 32 only instructions the rest of the > > > > image can perfectly contain 64bit code. Right now 64bit binary cannot > > > > be run with "qemu-system-x86_64 -kernel". But the same binary runs > > > > fine if packed with GRUB as iso. > > > > > > > > I tried to hack around this restriction by adding > > > > "OUTPUT_FORMAT(elf32-i386)" to the linker file and compiling project > > > > with 64bit support. But GNU ld program crashed at Ubuntu 14.04. It > > > > means not that many people use this code path. GNU ld compiled from > > > > HEAD does not have this problem but now GDB is confused by the fact > > > > that ELF contains 64bit code while header reports i386. > > > > That's unfortunate. > > > > > > Practically there is no reason for this check as it prevents running > > > > 64bit binaries with "qemu-system-x86_64 -kernel". > > > > One reason for the check is that it prevents that one loads a 64bit ELF > > binary that then fails strangely because it does not have the magic > > 32bit code to set up things. > > I would not call the 32bit preamble a magic - it is fairly well > documented; it is just a result of backward compatibility that > AMD/Intel carry for a long time. Every single 64-bit operation system > (Linux, Windows, FreeBSD, ...) has such 32bit init sequence.
FWIW, Anatol's patch also makes it possible for qemu to load the 64-bit Xen XTF tests. It would be a useful to at least have a way to override the check, even if it's still present by default.
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