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

[Stefan Hajnoczi]

Or instead of using linker behavior, maybe just change the #ifdef so it only applies when KVM is disabled. I didn't look at the code to see if this is possible, but it would be nice to avoid the very specific #ifdef condition in this patch.

Stefan

On Mon, Sep 11, 2023, 07:15 Stefan Hajnoczi <stefanha@gmail.com> wrote:

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(-)
>
> diff --git a/target/i386/kvm/kvm_i386.h b/target/i386/kvm/kvm_i386.h
> index 55d4e68c34..0b62ac628f 100644
> --- a/target/i386/kvm/kvm_i386.h
> +++ b/target/i386/kvm/kvm_i386.h
> @@ -32,7 +32,6 @@
>
>  bool kvm_has_smm(void);
>  bool kvm_enable_x2apic(void);
> -bool kvm_hv_vpindex_settable(void);
>  bool kvm_has_pit_state2(void);
>
>  bool kvm_enable_sgx_provisioning(KVMState *s);
> @@ -41,8 +40,6 @@ bool kvm_hyperv_expand_features(X86CPU *cpu, Error **errp);
>  void kvm_arch_reset_vcpu(X86CPU *cs);
>  void kvm_arch_after_reset_vcpu(X86CPU *cpu);
>  void kvm_arch_do_init_vcpu(X86CPU *cs);
> -uint32_t kvm_arch_get_supported_cpuid(KVMState *env, uint32_t function,
> -                                      uint32_t index, int reg);
>  uint64_t kvm_arch_get_supported_msr_feature(KVMState *s, uint32_t index);
>
>  void kvm_set_max_apic_id(uint32_t max_apic_id);
> @@ -60,6 +57,10 @@ void kvm_put_apicbase(X86CPU *cpu, uint64_t value);
>
>  bool kvm_has_x2apic_api(void);
>  bool kvm_has_waitpkg(void);
> +bool kvm_hv_vpindex_settable(void);
> +
> +uint32_t kvm_arch_get_supported_cpuid(KVMState *env, uint32_t function,
> +                                      uint32_t index, int reg);
>
>  uint64_t kvm_swizzle_msi_ext_dest_id(uint64_t address);
>  void kvm_update_msi_routes_all(void *private, bool global,
> @@ -76,6 +77,20 @@ typedef struct kvm_msr_handlers {
>  bool kvm_filter_msr(KVMState *s, uint32_t msr, QEMURDMSRHandler *rdmsr,
>                      QEMUWRMSRHandler *wrmsr);
>
> +#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.

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>