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

[Paolo Bonzini <pbonzini@redhat.com>]

On 06/12/19 11:23, Catherine Ho wrote:
> 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 <pbonzini@redhat.com>
> Signed-off-by: Catherine Ho <catherine.hecx@gmail.com>
> ---
>  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)
>                                           f[FEAT_VMX_SECONDARY_CTLS]));
>      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) {
> 

Yang Zhong from Intel also sent a similar patch.  Thanks very much to
both of you.

Paolo