[RFC PATCH v4 2/3] target/i386: Restrict system-specific features from user emulation

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

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>
---
RFC: I might have been overzealous in x86_cpu_expand_features(),
please double check.
---
 target/i386/cpu.c | 47 +++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 45 insertions(+), 2 deletions(-)

diff --git a/target/i386/cpu.c b/target/i386/cpu.c
index 760428d4dc..8b57708604 100644
--- a/target/i386/cpu.c
+++ b/target/i386/cpu.c
@@ -1664,6 +1664,7 @@ static inline uint64_t x86_cpu_xsave_xss_components(X86CPU *cpu)
            cpu->env.features[FEAT_XSAVE_XSS_LO];
 }
 
+#ifndef CONFIG_USER_ONLY
 /*
  * Returns the set of feature flags that are supported and migratable by
  * QEMU, for a given FeatureWord.
@@ -1686,6 +1687,7 @@ static uint64_t x86_cpu_get_migratable_flags(FeatureWord w)
     }
     return r;
 }
+#endif /* !CONFIG_USER_ONLY */
 
 void host_cpuid(uint32_t function, uint32_t count,
                 uint32_t *eax, uint32_t *ebx, uint32_t *ecx, uint32_t *edx)
@@ -5677,8 +5679,6 @@ CpuDefinitionInfoList *qmp_query_cpu_definitions(Error **errp)
     return cpu_list;
 }
 
-#endif /* !CONFIG_USER_ONLY */
-
 uint64_t x86_cpu_get_supported_feature_word(FeatureWord w,
                                             bool migratable_only)
 {
@@ -5781,6 +5781,38 @@ static void x86_cpu_get_cache_cpuid(uint32_t func, uint32_t index,
     }
 }
 
+#else /* CONFIG_USER_ONLY */
+
+uint64_t x86_cpu_get_supported_feature_word(FeatureWord w,
+                                            bool migratable_only)
+{
+    FeatureWordInfo *wi = &feature_word_info[w];
+
+    return wi->tcg_features;
+}
+
+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)
+{
+    *eax = 0;
+    *ebx = 0;
+    *ecx = 0;
+    *edx = 0;
+}
+
+static void x86_cpu_get_cache_cpuid(uint32_t func, uint32_t index,
+                                    uint32_t *eax, uint32_t *ebx,
+                                    uint32_t *ecx, uint32_t *edx)
+{
+    *eax = 0;
+    *ebx = 0;
+    *ecx = 0;
+    *edx = 0;
+}
+
+#endif /* !CONFIG_USER_ONLY */
+
 /*
  * Only for builtin_x86_defs models initialized with x86_register_cpudef_types.
  */
@@ -6163,6 +6195,7 @@ 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 +6214,7 @@ 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];
@@ -6876,6 +6910,8 @@ static void mce_init(X86CPU *cpu)
     }
 }
 
+#ifndef CONFIG_USER_ONLY
+
 static void x86_cpu_adjust_level(X86CPU *cpu, uint32_t *min, uint32_t value)
 {
     if (*min < value) {
@@ -6948,6 +6984,8 @@ static void x86_cpu_enable_xsave_components(X86CPU *cpu)
     env->features[FEAT_XSAVE_XSS_HI] = mask >> 32;
 }
 
+#endif /* !CONFIG_USER_ONLY */
+
 /***** Steps involved on loading and filtering CPUID data
  *
  * When initializing and realizing a CPU object, the steps
@@ -7040,6 +7078,7 @@ void x86_cpu_expand_features(X86CPU *cpu, Error **errp)
         }
     }
 
+#ifndef CONFIG_USER_ONLY
     if (!kvm_enabled() || !cpu->expose_kvm) {
         env->features[FEAT_KVM] = 0;
     }
@@ -7111,6 +7150,8 @@ void x86_cpu_expand_features(X86CPU *cpu, Error **errp)
         return;
     }
 
+#endif /* !CONFIG_USER_ONLY */
+
     /* Set cpuid_*level* based on cpuid_min_*level, if not explicitly set */
     if (env->cpuid_level_func7 == UINT32_MAX) {
         env->cpuid_level_func7 = env->cpuid_min_level_func7;
@@ -7152,6 +7193,7 @@ 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 +7221,7 @@ 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