[Changing subject to reflect reality]
I don't really understand what it is all about, as I initially said. But I've been told on freebsd IRC channels to post the issue to freebsd-emulation list, which I'm Cc'ing now, and if noone there answers, also to John Baldwin jhb@FreeBSD.
To bring some context back, here are a few pointers:
http://bugs.debian.org/737142 -- talking about pc-bsd, but it appears that the prob affects other BSD kernels too http://thread.gmane.org/gmane.comp.emulators.qemu/254074 -- this thread http://lists.gnu.org/archive/html/qemu-stable/2013-03/msg00037.html - a weird issue with freebsd reported last december
If it's a bug in *BSD kernels, maybe someone there will be glad to fix it. And maybe together we can come out with a more elegant solution to this issue, or maybe to confirm (or deny) that the proposed patch is a right thing to do.
Thanks,
/mjt
06.02.2014 08:04, Kevin O'Connor wrote:
On Wed, Feb 05, 2014 at 12:44:06PM -0500, Kevin O'Connor wrote:
On Tue, Feb 04, 2014 at 04:33:19PM +0400, Michael Tokarev wrote:
We have a bugreport in debian, http://bugs.debian.org/737142, stating that PC-BSD does not work with seabios-1.7.4 anymore.
I digged in, and found out that it fails only with -vga std (cirrus works fine). So I bisected the issue - only changing vgabios-stdvga.bin, and found this:
[...]
This seems to be similar to the freebsd problem reported last year - see: http://lists.gnu.org/archive/html/qemu-stable/2013-03/msg00037.html
[...]
(*) The best I can think of is to try and construct some hand crafted assembler that can catch buggy x86emu emulators and force a failure..
I have put together some assember to try and run-time test for known broken versions of x86emu. This patch to SeaVGABIOS does seem to catch the freebsd issue and in my tests it convinces freebsd to take a different approach. The patch is a bit ugly though.
Comments welcome. -Kevin
commit e3caa553e940efb6184b30a5637134c4aa9f8b65 Author: Kevin O'Connor kevin@koconnor.net Date: Wed Feb 5 22:47:29 2014 -0500
vgabios: Attempt to detect old x86emu and force a fault. Check for cases where the leal instruction does not work. This instruction is known to not be emulated properly on old versions of x86emu. If a broken version of x86emu is found, force a fault that x86emu will easily detect. This should help prevent soft failures when running old software. Signed-off-by: Kevin O'Connor <kevin@koconnor.net>diff --git a/vgasrc/vgaentry.S b/vgasrc/vgaentry.S index 9854448..e246e7c 100644 --- a/vgasrc/vgaentry.S +++ b/vgasrc/vgaentry.S @@ -45,9 +45,27 @@ _rom_header_signature:
- Entry points
****************************************************************/
// This macro is the same as ENTRY_ARG except the "calll"// instruction is avoided to work around known issues in the// emulation of some versions of x86emu.
// Force a fault if found to be running on broken x86emu versions.DECLFUNC x86emu_fault+x86emu_fault:
int $0x03+1: hlt
jmp 1b// This macro implements a call while avoiding instructions// that old versions of x86emu have problems with..macro VGA_CALLL cfunc// Make sure leal instruction works.movl $0x8000, %ecxleal (%ecx, %ecx, 1), %ecxcmpl $0x10000, %ecxjne x86emu_fault// Use callw instead of calllpush %axcallw \cfunc.endm// This macro is the same as ENTRY_ARG except VGA_CALLL is used. .macro ENTRY_ARG_VGA cfunc cli cld@@ -57,7 +75,7 @@ _rom_header_signature: movl %esp, %ebx // Backup %esp, then zero high bits movzwl %sp, %esp movl %esp, %eax // First arg is pointer to struct bregs
pushw %ax ; callw \cfunc
VGA_CALLL \cfunc movl %ebx, %esp // Restore %esp (including high bits) POPBREGS .endm@@ -103,7 +121,7 @@ entry_10_extrastack: movw %ds, %dx // Setup %ss/%esp and call function movw %dx, %ss movl %eax, %esp
pushw %ax ; callw handle_10
VGA_CALLL handle_10 movl %esp, %eax // Restore registers and return movw BREGS_size+4(%eax), %ss