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[PATCH] target/i386: skip kvm_msr_entry_add when kvm_vmx_basic is 0

From: Catherine Ho
Subject: [PATCH] target/i386: skip kvm_msr_entry_add when kvm_vmx_basic is 0
Date: Fri, 6 Dec 2019 05:23:37 -0500

Commit 1389309c811b ("KVM: nVMX: expose VMX capabilities for nested
hypervisors to userspace") expands the msr_based_features with
MSR_IA32_VMX_BASIC and others. Then together with an old kernel before
1389309c811b, the qemu call KVM_GET_MSR_FEATURE_INDEX_LIST and got the
smaller kvm_feature_msrs. Then in kvm_arch_get_supported_msr_feature(),
searching VMX_BASIC will be failed and return 0. At last kvm_vmx_basic
will be assigned to 0.

Without this patch, it will cause a qemu crash (host kernel 4.15
ubuntu 18.04+qemu 4.1):
qemu-system-x86_64: error: failed to set MSR 0x480 to 0x0
target/i386/kvm.c:2932: kvm_put_msrs: Assertion `ret ==
cpu->kvm_msr_buf->nmsrs' failed.

This fixes it by skipping kvm_msr_entry_add when kvm_vmx_basic is 0

Cc: Paolo Bonzini <address@hidden>
Signed-off-by: Catherine Ho <address@hidden>
 target/i386/kvm.c |    7 ++++++-
 1 files changed, 6 insertions(+), 1 deletions(-)

diff --git a/target/i386/kvm.c b/target/i386/kvm.c
index a8c44bf..8cf84a2 100644
--- a/target/i386/kvm.c
+++ b/target/i386/kvm.c
@@ -2632,8 +2632,13 @@ static void kvm_msr_entry_add_vmx(X86CPU *cpu, 
FeatureWordArray f)
     kvm_msr_entry_add(cpu, MSR_IA32_VMX_EPT_VPID_CAP,
                       f[FEAT_VMX_EPT_VPID_CAPS] | fixed_vmx_ept_vpid);
-    kvm_msr_entry_add(cpu, MSR_IA32_VMX_BASIC,
+    if (kvm_vmx_basic) {
+       /* Only add the entry when host supports it */
+        kvm_msr_entry_add(cpu, MSR_IA32_VMX_BASIC,
                       f[FEAT_VMX_BASIC] | fixed_vmx_basic);
+    }
     kvm_msr_entry_add(cpu, MSR_IA32_VMX_MISC,
                       f[FEAT_VMX_MISC] | fixed_vmx_misc);
     if (has_msr_vmx_vmfunc) {

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