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. This was previously documented in commit 2140cfa51d
("i386: Fix build by providing stub kvm_arch_get_supported_cpuid()").

Fix by partially reverting those commits, restoring a pair of stubs
for the unoptimized Clang builds.

Reported-by: Kevin Wolf <kwolf@redhat.com>
Suggested-by: Daniel P. Berrangé <berrange@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-stub.c  | 31 +++++++++++++++++++++++++++++++
 target/i386/kvm/meson.build |  2 ++
 2 files changed, 33 insertions(+)
 create mode 100644 target/i386/kvm/kvm-stub.c

diff --git a/target/i386/kvm/kvm-stub.c b/target/i386/kvm/kvm-stub.c
new file mode 100644
index 0000000000..d3d4a238ce
--- /dev/null
+++ b/target/i386/kvm/kvm-stub.c
@@ -0,0 +1,31 @@
+/*
+ * QEMU KVM x86 specific function stubs
+ *
+ * Copyright Linaro Limited 2012
+ *
+ * Author: Peter Maydell <peter.maydell@linaro.org>
+ *
+ * This work is licensed under the terms of the GNU GPL, version 2 or later.
+ * See the COPYING file in the top-level directory.
+ *
+ */
+#include "qemu/osdep.h"
+#include "cpu.h"
+#include "kvm_i386.h"
+
+#ifndef __OPTIMIZE__
+/* This function is only called inside conditionals which we
+ * rely on the compiler to optimize out when CONFIG_KVM is not
+ * defined.
+ */
+uint32_t kvm_arch_get_supported_cpuid(KVMState *env, uint32_t function,
+                                      uint32_t index, int reg)
+{
+    abort();
+}
+#endif
+
+bool kvm_hv_vpindex_settable(void)
+{
+    return false;
+}
diff --git a/target/i386/kvm/meson.build b/target/i386/kvm/meson.build
index 5d9174bbb5..40fbde96ca 100644
--- a/target/i386/kvm/meson.build
+++ b/target/i386/kvm/meson.build
@@ -1,3 +1,5 @@
+i386_ss.add(when: 'CONFIG_KVM', if_false: files('kvm-stub.c'))
+
 i386_softmmu_kvm_ss = ss.source_set()
 
 i386_softmmu_kvm_ss.add(files(
-- 
2.41.0

On 11/9/23 15:15, Philippe Mathieu-Daudé 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. This was previously documented in commit 2140cfa51d
> ("i386: Fix build by providing stub kvm_arch_get_supported_cpuid()").
> 
> Fix by partially reverting those commits, restoring a pair of stubs
> for the unoptimized Clang builds.
> 
> Reported-by: Kevin Wolf <kwolf@redhat.com>
> Suggested-by: Daniel P. Berrangé <berrange@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-stub.c  | 31 +++++++++++++++++++++++++++++++
>   target/i386/kvm/meson.build |  2 ++
>   2 files changed, 33 insertions(+)
>   create mode 100644 target/i386/kvm/kvm-stub.c

Patch superseded, see v3:
https://lore.kernel.org/qemu-devel/20230911142729.25548-1-philmd@linaro.org/