[Qemu-devel] Commit b2a575a1c652 broke i486 support.

[Qemu-devel] Commit b2a575a1c652 broke i486 support.

[Rob Landley]

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.

Rob

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

[Thomas Huth]

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).

 Thomas

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

[Richard W.M. Jones]

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
 
 # Drop -fstack-protector and the like
 QEMU_CFLAGS := $(filter -W%, $(QEMU_CFLAGS)) $(CFLAGS_NOPIE) -ffreestanding


Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
virt-builder quickly builds VMs from scratch
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Re: [Qemu-devel] Commit b2a575a1c652 broke i486 support.

[Thomas Huth]

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

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

[Rob Landley]

On 05/29/2017 05:14 AM, Richard W.M. Jones wrote:
> 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.

Is there any way to make it just _not_ load the option rom for -cpu 486?
It ran fine before that thing went in...

Rob

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

[Thomas Huth]

On 30.05.2017 21:05, Rob Landley wrote:
> On 05/29/2017 05:14 AM, Richard W.M. Jones wrote:
>> 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.
> 
> Is there any way to make it just _not_ load the option rom for -cpu 486?
> It ran fine before that thing went in...

Looking at that commit b2a575a1c652 that you've mentioned, it seems like
you could work-around the problem by disabling DMA for the fw_cfg
device. Try something like adding the parameter

 -global fw_cfg.dma_enabled=false

to your command line. If that does not help, try to use an older machine
type, like "-M pc-i440fx-2.5".

 Thomas

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

[Richard W.M. Jones]

On Tue, May 30, 2017 at 02:05:15PM -0500, Rob Landley wrote:
> On 05/29/2017 05:14 AM, Richard W.M. Jones wrote:
> > 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.
> 
> Is there any way to make it just _not_ load the option rom for -cpu 486?
> It ran fine before that thing went in...

Well it ran, but it was very slow.  The DMA method can be literally
orders of magnitude faster.

Does adding the flag I suggested help?

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
virt-builder quickly builds VMs from scratch
http://libguestfs.org/virt-builder.1.html

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

[Paolo Bonzini]

On 30/05/2017 21:05, Rob Landley wrote:
> On 05/29/2017 05:14 AM, Richard W.M. Jones wrote:
>> 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.
> 
> Is there any way to make it just _not_ load the option rom for -cpu 486?
> It ran fine before that thing went in...

You were loading another option ROM.  Thomas's suggested -global
invocation will revert to that ROM until we fix it.

Paolo