Re: [Qemu-devel] [PATCH uq/master] kvm: x86: Separately write feature control MSR on reset

[Paolo Bonzini]

Il 17/12/2013 20:05, Jan Kiszka ha scritto:
> If the guest is running in nested mode on system reset, clearing the
> feature MSR signals the kernel to leave this mode. Recent kernels
> processes this properly, but leave the VCPU state undefined behind. It
> is the job of userspace to bring it to a proper shape. Therefore, write
> this specific MSR first so that no state transfer gets lost.
> 
> This allows to cleanly reset a guest with VMX in use.
> 
> Signed-off-by: Jan Kiszka <address@hidden>
> ---
>  target-i386/kvm.c | 32 ++++++++++++++++++++++++++++----
>  1 file changed, 28 insertions(+), 4 deletions(-)
> 
> diff --git a/target-i386/kvm.c b/target-i386/kvm.c
> index 1188482..ec51447 100644
> --- a/target-i386/kvm.c
> +++ b/target-i386/kvm.c
> @@ -1104,6 +1104,25 @@ static int kvm_put_tscdeadline_msr(X86CPU *cpu)
>      return kvm_vcpu_ioctl(CPU(cpu), KVM_SET_MSRS, &msr_data);
>  }
>  
> +/*
> + * Provide a separate write service for the feature control MSR in order to
> + * kick the VCPU out of VMXON or even guest mode on reset. This has to be 
> done
> + * before writing any other state because forcibly leaving nested mode
> + * invalidates the VCPU state.
> + */
> +static int kvm_put_msr_feature_control(X86CPU *cpu)
> +{
> +    struct {
> +        struct kvm_msrs info;
> +        struct kvm_msr_entry entry;
> +    } msr_data;
> +
> +    kvm_msr_entry_set(&msr_data.entry, MSR_IA32_FEATURE_CONTROL,
> +                      cpu->env.msr_ia32_feature_control);
> +    msr_data.info.nmsrs = 1;
> +    return kvm_vcpu_ioctl(CPU(cpu), KVM_SET_MSRS, &msr_data);
> +}
> +
>  static int kvm_put_msrs(X86CPU *cpu, int level)
>  {
>      CPUX86State *env = &cpu->env;
> @@ -1204,10 +1223,8 @@ static int kvm_put_msrs(X86CPU *cpu, int level)
>          if (cpu->hyperv_vapic) {
>              kvm_msr_entry_set(&msrs[n++], HV_X64_MSR_APIC_ASSIST_PAGE, 0);
>          }
> -        if (has_msr_feature_control) {
> -            kvm_msr_entry_set(&msrs[n++], MSR_IA32_FEATURE_CONTROL,
> -                              env->msr_ia32_feature_control);
> -        }
> +        /* Note: MSR_IA32_FEATURE_CONTROL is written separately, see
> +         *       kvm_put_msr_feature_control. */
>      }
>      if (env->mcg_cap) {
>          int i;
> @@ -1801,6 +1818,13 @@ int kvm_arch_put_registers(CPUState *cpu, int level)
>  
>      assert(cpu_is_stopped(cpu) || qemu_cpu_is_self(cpu));
>  
> +    if (level >= KVM_PUT_RESET_STATE && has_msr_feature_control) {
> +        ret = kvm_put_msr_feature_control(x86_cpu);
> +        if (ret < 0) {
> +            return ret;
> +        }
> +    }
> +
>      ret = kvm_getput_regs(x86_cpu, 1);
>      if (ret < 0) {
>          return ret;
> 

Applied, thanks!