Re: [Qemu-devel] [PATCH] target-i386: Enhance the stub for kvm_arch_get_supported_cpuid()

[Kamil Rytarowski]

On 14.02.2019 21:51, Paolo Bonzini wrote:
> On 14/02/19 20:41, Kamil Rytarowski wrote:
>> Please do replace the current kludge that is sensitive to:
>>  - compiler behavior that can change with new versions
>>  - compiler gcc/clang
>>  - optimization options
> 
> Not really, any half-decent compiler will optimize away "if (0)" and
> QEMU is far from being the only software that relies on that.
> 
> GCC has been doing that even at -O0 for like 15 years, at some point it
> was basically the only optimization it did.  Just try it for yourself:
> 
>       int f(void);
> 
>       int main()
>       {
>               if (0)
>                       return f();
>               else
>                       return 0;
>       }
> 
> Throw it at all compilers and optimization levels, and it *will* work.
> If it doesn't then I'll consider again your patch.
> 

I consider it as fragile hack and certainly not something to depend on.
Also in some circumstances of such code, especially "if (zero0)" we want
to enable disabled code under a debugger.

There were also kernel backdoors due to this optimization.

>>  - linux(KVM) - non-linux (no-KVM) build
> 
> That's the point.  We want your non-Linux non-KVM build to be as lean as
> possible and not cause possible run-time failures due to people
> forgetting about them.
> 
>>  - community not actively testing non-linux no-kvm build with
>> optimization on clang
> 
> False, we test OS X and there are VM builds for the BSDs.

Unfortunately not in the same combination of options as nobody caught it
in years. (Probably not many people actually develop it on these OSes
with debug flags). I was keeping this patch locally for some time now.
This hack was introduced several years ago.

>> My patch replaced it makes it work.
>>
>> Build error:
>>
>>   LINK    i386-bsd-user/qemu-i386
> 
> Ok, please use "make -C i386-bsd-user target/i386/cpu.o V=1" to get the
> command line, invoke it again with "-save-temps" at the end, and send me
> both the command line and the resulting "cpu.i" file.
> 

I'm building qemu with pkgsrc that provides all the dependencies and
compiler settings. It also uses wrappers to translate original compiler
options with transformed ones.

Log from pkgsrc with command lines:

http://netbsd.org/~kamil/qemu/qemu-build-2019-02-14.txt.bz2

Requested cpu.i (hopefully correctly generated)

http://netbsd.org/~kamil/qemu/cpu.i.bz2

I've generated it manually with this command.

/usr/local/bin/clang -iquote
/tmp/pkgsrc-tmp/wip/qemu-haxm/work/qemu-3.0.0/target/i386 -iquote
target/i386 -iquote /tmp/pkgsrc-tmp/wip/qemu-haxm/work/qemu-3.0.0/tcg
-iquote /tmp/pkgsrc-tmp/wip/qemu-haxm/work/qemu-3.0.0/tcg/i386 -iquote .
-iquote /tmp/pkgsrc-tmp/wip/qemu-haxm/work/qemu-3.0.0 -iquote
/tmp/pkgsrc-tmp/wip/qemu-haxm/work/qemu-3.0.0/accel/tcg -iquote
/tmp/pkgsrc-tmp/wip/qemu-haxm/work/qemu-3.0.0/include
-I/tmp/pkgsrc-tmp/wip/qemu-haxm/work/.x11-buildlink/include/pixman-1
-I/tmp/pkgsrc-tmp/wip/qemu-haxm/work/qemu-3.0.0/dtc/libfdt -pthread
-I/tmp/pkgsrc-tmp/wip/qemu-haxm/work/.buildlink/include/glib/glib-2.0
-I/tmp/pkgsrc-tmp/wip/qemu-haxm/work/.buildlink/lib/glib-2.0/include
-I/tmp/pkgsrc-tmp/wip/qemu-haxm/work/.buildlink/include -m64 -mcx16
-D_GNU_SOURCE -D_FILE_OFFSET_BITS=64 -D_LARGEFILE_SOURCE
-Wstrict-prototypes -Wredundant-decls -Wall -Wundef -Wwrite-strings
-Wmissing-prototypes -fno-strict-aliasing -fno-common -fwrapv
-Wno-error=address-of-packed-member -Wno-string-plus-int
-Wno-initializer-overrides -Wexpansion-to-defined -Wendif-labels
-Wno-shift-negative-value -Wno-missing-include-dirs -Wempty-body
-Wnested-externs -Wformat-security -Wformat-y2k -Winit-self
-Wignored-qualifiers -Wold-style-definition -Wtype-limits
-fstack-protector-strong
-I/tmp/pkgsrc-tmp/wip/qemu-haxm/work/.buildlink/include/libpng16
-I/tmp/pkgsrc-tmp/wip/qemu-haxm/work/qemu-3.0.0/capstone/include -iquote
.. -iquote /tmp/pkgsrc-tmp/wip/qemu-haxm/work/qemu-3.0.0/target/i386
-DNEED_CPU_H -iquote
/tmp/pkgsrc-tmp/wip/qemu-haxm/work/qemu-3.0.0/include -MMD -MP -MT
target/i386/cpu.o -MF target/i386/cpu.d -O2 -g -O2 -O0 -g -ggdb
-I/tmp/pkgsrc-tmp/wip/qemu-haxm/work/.buildlink/include/SDL2
-I/tmp/pkgsrc-tmp/wip/qemu-haxm/work/.x11-buildlink/include
-I/tmp/pkgsrc-tmp/wip/qemu-haxm/work/.x11-buildlink/include/libdrm
-I/tmp/pkgsrc-tmp/wip/qemu-haxm/work/.buildlink/include/glib/gio-unix-2.0
-I/usr/include/krb5 -c -o target/i386/cpu.o
/tmp/pkgsrc-tmp/wip/qemu-haxm/work/qemu-3.0.0/target/i386/cpu.c
-Qunused-arguments -fstack-protector -save-temps

> Paolo
> 
>> /usr/bin/ld: /usr/lib/libc.so and /usr/lib/crt0.o: warning: multiple
>> common of `environ'
>> /usr/bin/ld: target/i386/cpu.o: in function `x86_cpu_filter_features':
>> /tmp/pkgsrc-tmp/emulators/qemu/work/qemu-3.1.0/target/i386/cpu.c:5047:
>> undefined reference to `kvm_arch_get_supported_cpuid'
>> /usr/bin/ld:
>> /tmp/pkgsrc-tmp/emulators/qemu/work/qemu-3.1.0/target/i386/cpu.c:5048:
>> undefined reference to `kvm_arch_get_supported_cpuid'
>> /usr/bin/ld:
>> /tmp/pkgsrc-tmp/emulators/qemu/work/qemu-3.1.0/target/i386/cpu.c:5049:
>> undefined reference to `kvm_arch_get_supported_cpuid'
>> /usr/bin/ld:
>> /tmp/pkgsrc-tmp/emulators/qemu/work/qemu-3.1.0/target/i386/cpu.c:5050:
>> undefined reference to `kvm_arch_get_supported_cpuid'
>> /usr/bin/ld:
>> /tmp/pkgsrc-tmp/emulators/qemu/work/qemu-3.1.0/target/i386/cpu.c:5051:
>> undefined reference to `kvm_arch_get_supported_cpuid'
>> clang-9: error: linker command failed with exit code 1 (use -v to see
>> invocation)
>> make[1]: *** [Makefile:199: qemu-i386] Error 1
>> gmake: *** [Makefile:483: subdir-i386-bsd-user] Error 2
>> gmake: *** Waiting for unfinished jobs....
>>   LINK    x86_64-bsd-user/qemu-x86_64
>> /usr/bin/ld: /usr/lib/libc.so and /usr/lib/crt0.o: warning: multiple
>> common of `environ'
>> /usr/bin/ld: target/i386/cpu.o: in function `x86_cpu_filter_features':
>> /tmp/pkgsrc-tmp/emulators/qemu/work/qemu-3.1.0/target/i386/cpu.c:5047:
>> undefined reference to `kvm_arch_get_supported_cpuid'
>> /usr/bin/ld:
>> /tmp/pkgsrc-tmp/emulators/qemu/work/qemu-3.1.0/target/i386/cpu.c:5048:
>> undefined reference to `kvm_arch_get_supported_cpuid'
>> /usr/bin/ld:
>> /tmp/pkgsrc-tmp/emulators/qemu/work/qemu-3.1.0/target/i386/cpu.c:5049:
>> undefined reference to `kvm_arch_get_supported_cpuid'
>> /usr/bin/ld:
>> /tmp/pkgsrc-tmp/emulators/qemu/work/qemu-3.1.0/target/i386/cpu.c:5050:
>> undefined reference to `kvm_arch_get_supported_cpuid'
>> /usr/bin/ld:
>> /tmp/pkgsrc-tmp/emulators/qemu/work/qemu-3.1.0/target/i386/cpu.c:5051:
>> undefined reference to `kvm_arch_get_supported_cpuid'
>> clang-9: error: linker command failed with exit code 1 (use -v to see
>> invocation)
>> make[1]: *** [Makefile:199: qemu-x86_64] Error 1
>> gmake: *** [Makefile:483: subdir-x86_64-bsd-user] Error 2
>> *** Error code 2
>>
> 
>

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