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>
---
 target/i386/cpu.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/target/i386/cpu.c b/target/i386/cpu.c
index 24ee67b42d..83914d5d1b 100644
--- a/target/i386/cpu.c
+++ b/target/i386/cpu.c
@@ -6163,6 +6163,7 @@ void cpu_x86_cpuid(CPUX86State *env, uint32_t index, uint32_t count,
             }
             *edx = env->features[FEAT_7_0_EDX]; /* Feature flags */
 
+#ifndef CONFIG_USER_ONLY
             /*
              * SGX cannot be emulated in software.  If hardware does not
              * support enabling SGX and/or SGX flexible launch control,
@@ -6181,6 +6182,7 @@ void cpu_x86_cpuid(CPUX86State *env, uint32_t index, uint32_t count,
                     CPUID_7_0_ECX_SGX_LC))) {
                 *ecx &= ~CPUID_7_0_ECX_SGX_LC;
             }
+#endif
         } else if (count == 1) {
             *eax = env->features[FEAT_7_1_EAX];
             *edx = env->features[FEAT_7_1_EDX];
@@ -7152,6 +7154,7 @@ static void x86_cpu_filter_features(X86CPU *cpu, bool verbose)
         mark_unavailable_features(cpu, w, unavailable_features, prefix);
     }
 
+#ifndef CONFIG_USER_ONLY
     if ((env->features[FEAT_7_0_EBX] & CPUID_7_0_EBX_INTEL_PT) &&
         kvm_enabled()) {
         KVMState *s = CPU(cpu)->kvm_state;
@@ -7179,6 +7182,7 @@ static void x86_cpu_filter_features(X86CPU *cpu, bool verbose)
             mark_unavailable_features(cpu, FEAT_7_0_EBX, CPUID_7_0_EBX_INTEL_PT, prefix);
         }
     }
+#endif
 }
 
 static void x86_cpu_hyperv_realize(X86CPU *cpu)
-- 
2.41.0

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

Reviewed-by: Michael Tokarev <mjt@tls.msk.ru>

Thank you for patience!

11.09.2023 17:27, Philippe Mathieu-Daudé:
> 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
...