i386: Mask SVM features if nested SVM is disabled

QEMU incorrectly validates FEAT_SVM feature flags against
GET_SUPPORTED_CPUID even if SVM features are being masked out by
cpu_x86_cpuid().  This can make QEMU print warnings on most AMD
CPU models, even when SVM nesting is disabled (which is the
default).

This bug was never detected before because of a Linux KVM bug:
until Linux v5.6, KVM was not filtering out SVM features in
GET_SUPPORTED_CPUID when nested was disabled.  This KVM bug was
fixed in Linux v5.7-rc1, on Linux commit a50718cc3f43 ("KVM:
nSVM: Expose SVM features to L1 iff nested is enabled").

Fix the problem by adding a CPUID_EXT3_SVM dependency to all
FEAT_SVM feature flags in the feature_dependencies table.

Reported-by: Yanan Fu <[email protected]>
Signed-off-by: Eduardo Habkost <[email protected]>
Message-Id: <[email protected]>
[Fix testcase. - Paolo]
Signed-off-by: Paolo Bonzini <[email protected]>

Author: ehabkost

target/i386/cpu.c

@@ -1404,6 +1404,10 @@ static FeatureDep feature_dependencies[] = {
         .from = { FEAT_VMX_SECONDARY_CTLS,  VMX_SECONDARY_EXEC_ENABLE_VMFUNC },
         .to = { FEAT_VMX_VMFUNC,            ~0ull },
     },
+    {
+        .from = { FEAT_8000_0001_ECX,       CPUID_EXT3_SVM },
+        .to = { FEAT_SVM,                   ~0ull },
+    },
 };
 
 typedef struct X86RegisterInfo32 {

tests/qtest/test-x86-cpuid-compat.c

@@ -256,7 +256,7 @@ int main(int argc, char **argv)
                    "-cpu 486,+invtsc", "xlevel", 0x80000007);
     /* CPUID[8000_000A].EDX: */
     add_cpuid_test("x86/cpuid/auto-xlevel/486/npt",
-                   "-cpu 486,+npt", "xlevel", 0x8000000A);
+                   "-cpu 486,+svm,+npt", "xlevel", 0x8000000A);
     /* CPUID[C000_0001].EDX: */
     add_cpuid_test("x86/cpuid/auto-xlevel2/phenom/xstore",
                    "-cpu phenom,+xstore", "xlevel2", 0xC0000001);
@@ -348,7 +348,7 @@ int main(int argc, char **argv)
                    "-machine pc-i440fx-2.4 -cpu SandyBridge,",
                    "xlevel", 0x80000008);
     add_cpuid_test("x86/cpuid/xlevel-compat/pc-i440fx-2.4/npt-on",
-                   "-machine pc-i440fx-2.4 -cpu SandyBridge,+npt",
+                   "-machine pc-i440fx-2.4 -cpu SandyBridge,+svm,+npt",
                    "xlevel", 0x80000008);
 
     /* Test feature parsing */