Re: [PATCH] target/i386: Re-introduce few KVM stubs for Clang debug builds

["Philippe Mathieu-Daudé" <philmd@linaro.org>]

On 11/9/23 14:32, Kevin Wolf wrote:
> Am 11.09.2023 um 13:15 hat Stefan Hajnoczi geschrieben:
>> On Mon, 11 Sept 2023 at 06:39, Philippe Mathieu-Daudé <philmd@linaro.org> wrote:
>>>
>>> Since commits 3adce820cf..ef1cf6890f, When building on
>>> a x86 host configured as:
>>>
>>>    $ ./configure --cc=clang \
>>>      --target-list=x86_64-linux-user,x86_64-softmmu \
>>>      --enable-debug
>>>
>>> we get:
>>>
>>>    [71/71] Linking target qemu-x86_64
>>>    FAILED: qemu-x86_64
>>>    /usr/bin/ld: libqemu-x86_64-linux-user.fa.p/target_i386_cpu.c.o: in function `cpu_x86_cpuid':
>>>    cpu.c:(.text+0x1374): undefined reference to `kvm_arch_get_supported_cpuid'
>>>    /usr/bin/ld: libqemu-x86_64-linux-user.fa.p/target_i386_cpu.c.o: in function `x86_cpu_filter_features':
>>>    cpu.c:(.text+0x81c2): undefined reference to `kvm_arch_get_supported_cpuid'
>>>    /usr/bin/ld: cpu.c:(.text+0x81da): undefined reference to `kvm_arch_get_supported_cpuid'
>>>    /usr/bin/ld: cpu.c:(.text+0x81f2): undefined reference to `kvm_arch_get_supported_cpuid'
>>>    /usr/bin/ld: cpu.c:(.text+0x820a): undefined reference to `kvm_arch_get_supported_cpuid'
>>>    /usr/bin/ld: libqemu-x86_64-linux-user.fa.p/target_i386_cpu.c.o:cpu.c:(.text+0x8225): more undefined references to `kvm_arch_get_supported_cpuid' follow
>>>    clang: error: linker command failed with exit code 1 (use -v to see invocation)
>>>    ninja: build stopped: subcommand failed.
>>>
>>> '--enable-debug' disables optimizations (CFLAGS=-O0).
>>>
>>> While at this (un)optimization level GCC eliminate the
>>> following dead code:
>>>
>>>    if (0 && foo()) {
>>>        ...
>>>    }
>>>
>>> Clang does not. Therefore restore a pair of stubs for
>>> unoptimized Clang builds.
>>>
>>> Reported-by: Kevin Wolf <kwolf@redhat.com>
>>> Fixes: 3adce820cf ("target/i386: Remove unused KVM stubs")
>>> Fixes: ef1cf6890f ("target/i386: Allow elision of kvm_hv_vpindex_settable()")
>>> Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
>>> ---
>>>   target/i386/kvm/kvm_i386.h | 21 ++++++++++++++++++---
>>>   1 file changed, 18 insertions(+), 3 deletions(-)


>>> +#elif defined(__clang__) && !defined(__OPTIMIZE__)
>>
>> Another approach is a static library with a .o file containing the
>> stubs so the linker only includes it in the executable if the compiler
>> emitted the symbols. That way there is no need for defined(__clang__)
>> && !defined(__OPTIMIZE__) and it will work with other
>> compilers/optimization levels. It's more work to set up though.
> 
> Isn't that exactly how it was before the stubs were removed? It would be
> a simple revert of that commit.
> 
> The approach with static inline functions defined only for a very
> specific configuration looks a lot more fragile to me. In fact, I'm
> surprised that it works because I think it requires that the header
> isn't used in any files that are shared between user space and system
> emulation - and naively cpu.c sounded like something that could be
> shared. Looks like this patch only works because the linux-user target
> uses a separate build of the same CPU emulation source file.
> 
> So I think reverting the commit that removed the stubs would be much
> more obvious.

Yes, v2 reverts:
https://lore.kernel.org/qemu-devel/20230911131507.24943-1-philmd@linaro.org/