[Qemu-devel] Booting kernels with PVHVM documentation?

[Alex Bennée]

Hi,

I've been looking at using PVH as an alternative to a long bios boot
sequence to boot some x86_64 test kernels for tests/tcg. I'm finding it
hard to piece together all the bits but I naively thought it would just
be a case of adding a few ELF NOTES to my boot.S with something like:

          ELFNOTE(Xen, XEN_ELFNOTE_VIRT_BASE,      _ASM_PTR 0x100000)
          ELFNOTE(Xen, XEN_ELFNOTE_ENTRY,          _ASM_PTR _start)
          ELFNOTE(Xen, XEN_ELFNOTE_PHYS32_ENTRY,   _ASM_PTR 0)    /* entry == 
virtbase */
          ELFNOTE(Xen, XEN_ELFNOTE_PADDR_OFFSET,   _ASM_PTR 0)

          .code64
          .section .text
          /* Kernel Entry Point */
  .global _start
  _start:
          // Setup stack ASAP
          movq $stack_end,%rsp

However I'm running into lots of head scratching as the get_elf_note
code seems to skip over the notes before failing:

  ./qemu-system-x86_64 -monitor none -display none \
    -chardev stdio,id=out -device isa-debugcon,chardev=out \
    -device isa-debug-exit,iobase=0xf4,iosize=0x4 -kernel ./tests/hello
  load_elf64: processing hdr:0 of type 1
  load_elf64: processing hdr:1 of type 4
  get_elf_note_type64: looking for type 18, first is 3
  get_elf_note_type64: 4/20
  get_elf_note_type64: offset is 36
  get_elf_note_type64: note is 0
  get_elf_note_type64: 0/123713
  get_elf_note_type64: offset is 123728
  load_elf64: processing hdr:2 of type 1685382481
  qemu-system-x86_64: Error loading uncompressed kernel without PVH ELF Note

So I thought I'd go back to the Linux kernel and see if I could get it
to boot up. So I built an x86_64 kernel with:

  CONFIG_XEN_PVHVM=y
  CONFIG_XEN_PVHVM_SMP=y
  CONFIG_XEN_PVH=y
  CONFIG_PVH=y

And tried to boot that, it certainly gets a lot further but in detecting
the note 18 it's looking for but then doesn't provide any output. So I
started digging around the patches and saw talk of a PVH option ROM
which does all the x86 mode escalation before booting the kernel.
However I was unable to find any documentation about if I should be
adding this manually to my command line or if it is auto-magiced into
place. So I have a number of questions:

  * what's the canonical command line for booting a Linux PVHVM kernel?
  * should this work in TCG as well?
  * are they any special linker rules required for the Xen.notes?

And finally:

  * is this idea of mine a weird abuse of the PVHVM boot protocol or
    does it make sense?

Thanks in advance for any elucidation ;-)

--
Alex Bennée