Re: [Qemu-devel] Commit b2a575a1c652 broke i486 support.

[Thomas Huth <thuth@redhat.com>]

On 29.05.2017 12:14, Richard W.M. Jones wrote:
> On Mon, May 29, 2017 at 11:22:14AM +0200, Thomas Huth wrote:
>> On 28.05.2017 07:18, Rob Landley wrote:
>>> You can't boot a kernel under -cpu 486 since that commit (hangs
>>> producing no output) because it added a bios image that won't run on
>>> anything short of pentium II.
>>>
>>> You can try the run-emulator.sh script in
>>> http://landley.net/aboriginal/downloads/binaries/system-image-i486.tar.gz
>>> before and after that commit to check for yourself.
>>
>>  Hi Rob,
>>
>> thanks for the bug report ... but to make sure that your mail does not
>> get lost in the high traffic of the qemu-devel mailing list, it might be
>> a good idea to CC: the people from that commit b2a575a1c652 in this case
>> (which I've done now).
> 
> I see in the disassembly use of cmovne (new in Pentium Pro) and
> bswap (new in 486).
> [http://cse.unl.edu/~goddard/Courses/CSCE351/IntelArchitecture/InstructionSetSummary.pdf]
> 
> The cmovne instruction is generated by the compiler (GCC in my case),
> 
> The following patch removes the cmovne instruction, so it should work
> on 486 (although I didn't test it).  It's not possible to remove bswap
> without surgery on the inline assembler.
> 
> --- a/pc-bios/optionrom/Makefile
> +++ b/pc-bios/optionrom/Makefile
> @@ -13,6 +13,7 @@ $(call set-vpath, $(SRC_PATH)/pc-bios/optionrom)
>  ifeq ($(lastword $(filter -O%, -O0 $(CFLAGS))),-O0)
>  override CFLAGS += -O2
>  endif
> +override CFLAGS += -march=i386

I guess you could also use "-march=i486" here instead, since QEMU does
not emulate a 386, as far as I can see.

 Thomas