Re: [PATCH v3] target/i386: Restrict system-specific features from user emulation

[Kevin Wolf <kwolf@redhat.com>]

Am 11.09.2023 um 16:27 hat Philippe Mathieu-Daudé geschrieben:
> Since commits 3adce820cf ("target/i386: Remove unused KVM
> stubs") and ef1cf6890f ("target/i386: Allow elision of
> kvm_hv_vpindex_settable()"), 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.
> 
> libqemu-x86_64-linux-user.fa is user emulation specific, so
> having system emulation code called there is dubious.
> 
> '--enable-debug' disables optimizations (CFLAGS=-O0).
> 
> While at this (un)optimization level GCC eliminate the
> following dead code (CPP output of mentioned build):
> 
>  static void x86_cpu_get_supported_cpuid(uint32_t func, uint32_t index,
>                                          uint32_t *eax, uint32_t *ebx,
>                                          uint32_t *ecx, uint32_t *edx)
>  {
>      if ((0)) {
>          *eax = kvm_arch_get_supported_cpuid(kvm_state, func, index, R_EAX);
>          *ebx = kvm_arch_get_supported_cpuid(kvm_state, func, index, R_EBX);
>          *ecx = kvm_arch_get_supported_cpuid(kvm_state, func, index, R_ECX);
>          *edx = kvm_arch_get_supported_cpuid(kvm_state, func, index, R_EDX);
>      } else if (0) {
>          *eax = 0;
>          *ebx = 0;
>          *ecx = 0;
>          *edx = 0;
>      } else {
>          *eax = 0;
>          *ebx = 0;
>          *ecx = 0;
>          *edx = 0;
>      }
>  }
> 
> Clang does not.
> 
> Instead of trying to deal with compiler specific checks around
> __OPTIMIZE__ (see commit 2140cfa51d "i386: Fix build by providing
> stub kvm_arch_get_supported_cpuid()"), simply restrict code
> belonging to system emulation, easing user emulation linking.
> 
> Reported-by: Kevin Wolf <kwolf@redhat.com>
> Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>

Can we make the function declarations in the header file for the
functions without stubs conditional on !CONFIG_USER_ONLY, too, so that
trying to call them would already fail during compilation (and also with
-O2), not only when linking without optimisations?

Kevin