Hi,
I'm trying to bootstrap a new PVH-only Xen domU OS "from scratch", to
replace our existing use of Mini-OS for the early boot/low-level support
layer in MirageOS. I've done this by creating new Xen bindings for Solo5
[1], basing them on our existing virtio code [2].
Unfortunately, I can't seem to get past the first few instructions on VCPU
boot. Here's what I have at the moment (abridged):
.section .note.solo5.xen
.align 4
.long 4
.long 4
.long XEN_ELFNOTE_PHYS32_ENTRY
.ascii "Xen\0"
.long _start32
/* ... */
.code32
ENTRY(_start32)
cld
lgdt (gdt64_ptr)
ljmp $0x10, $1f
1: movl $0x18, %eax
movl %eax, %ds
movl %eax, %es
movl %eax, %ss
xorl %eax, %eax
movl %eax, %fs
movl %eax, %gs
I have verified, via xl -v create -c ..., that the domain builder appears
to be doing the right thing, and is interpreting the ELF NOTE correctly.
However, for some reason I cannot fathom, I get a triple fault on the ljmp
following the lgdt instruction above:
(XEN) d31v0 Triple fault - invoking HVM shutdown action 1
(XEN) *** Dumping Dom31 vcpu#0 state: ***
(XEN) ----[ Xen-4.11.4-pre x86_64 debug=n Not tainted ]----
(XEN) CPU: 0
(XEN) RIP: 0000:[<0000000000100028>]
(XEN) RFLAGS: 0000000000010002 CONTEXT: hvm guest (d31v0)
(XEN) rax: 0000000000000000 rbx: 0000000000116000 rcx: 0000000000000000
(XEN) rdx: 0000000000000000 rsi: 0000000000000000 rdi: 0000000000000000
(XEN) rbp: 0000000000000000 rsp: 0000000000000000 r8: 0000000000000000
(XEN) r9: 0000000000000000 r10: 0000000000000000 r11: 0000000000000000
(XEN) r12: 0000000000000000 r13: 0000000000000000 r14: 0000000000000000
(XEN) r15: 0000000000000000 cr0: 0000000000000011 cr4: 0000000000000000
(XEN) cr3: 0000000000000000 cr2: 0000000000000000
(XEN) fsb: 0000000000000000 gsb: 0000000000000000 gss: 0000000000000000
(XEN) ds: 0000 es: 0000 fs: 0000 gs: 0000 ss: 0000 cs: 0000
Cross-checking 0x100028 via gdb:
Dump of assembler code for function _start32:
0x00100020 <+0>: cld
0x00100021 <+1>: lgdtl 0x108040
0x00100028 <+8>: ljmp $0x10,$0x10002f
0x0010002f <+15>: mov $0x18,%eax
I've spent a couple of days trying various things and cross-checking both
with the Mini-OS PVH/HVM startup [3] and the Intel SDM, but no joy. I've
also re-checked the GDT selector values used by the original virtio code
which this is based on, and they appear to be fine.
This is not helped by the fact that the Xen domU PVH start-of-day VCPU
state does not seem to be documented anywhere, with the exception of
"struct hvm_start_info is passed in %ebx" as stated in
arch-x86/hvm/start_info.h.
In case it's relevant, I'm testing with Xen 4.11.4 as shipped with Debian
10, on an Intel Broadwell CPU.
Any ideas? Any help much appreciated.
Thanks,
-mato
[1] https://github.com/mato/solo5/tree/xen/bindings/xen /
https://github.com/mato/solo5/commit/f2539d588883a2e8854998c75bdea9b10f113ed6
[2] https://github.com/mato/solo5/tree/xen/bindings/virtio
[3]
https://xenbits.xen.org/gitweb/?p=mini-os.git;a=blob;f=arch/x86/x86_hvm.S;h=6e8ad983a16adbe97b343f7dbc17e281ee0c389f;hb=HEAD
