Re: [PATCH v3 22/25] KVM: nVMX: Always set required-1 bits of pinbased_ctls to PIN_BASED_ALWAYSON_WITHOUT_TRUE_MSR

Maxim Levitsky
2022-07-12T12:02:27Z

On Fri, 2022-07-08 at 16:42 +0200, Vitaly Kuznetsov wrote:
> Similar to exit_ctls_low, entry_ctls_low, procbased_ctls_low,
> pinbased_ctls_low should be set to PIN_BASED_ALWAYSON_WITHOUT_TRUE_MSR
> and not host's MSR_IA32_VMX_PINBASED_CTLS value |=
> PIN_BASED_ALWAYSON_WITHOUT_TRUE_MSR.
>
> The commit eabeaaccfca0 ("KVM: nVMX: Clean up and fix pin-based
> execution controls") which introduced '|=' doesn't mention anything
> about why this is needed, the change seems rather accidental.
>
> Note: normally, required-1 portion of MSR_IA32_VMX_PINBASED_CTLS should
> be equal to PIN_BASED_ALWAYSON_WITHOUT_TRUE_MSR so no behavioral change
> is expected, however, it is (in theory) possible to observe something
> different there when e.g. KVM is running as a nested hypervisor. Hope
> this doesn't happen in practice.
>
> Reported-by: Jim Mattson <jmattson@google.com>
> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
> ---
>  arch/x86/kvm/vmx/nested.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> index dcf3ee645212..09654d5c2144 100644
> --- a/arch/x86/kvm/vmx/nested.c
> +++ b/arch/x86/kvm/vmx/nested.c
> @@ -6586,7 +6586,7 @@ void nested_vmx_setup_ctls_msrs(struct nested_vmx_msrs *msrs, u32 ept_caps)
>         rdmsr(MSR_IA32_VMX_PINBASED_CTLS,
>                 msrs->pinbased_ctls_low,
>                 msrs->pinbased_ctls_high);
> -       msrs->pinbased_ctls_low |=
> +       msrs->pinbased_ctls_low =
>                 PIN_BASED_ALWAYSON_WITHOUT_TRUE_MSR;
>         msrs->pinbased_ctls_high &=
>                 PIN_BASED_EXT_INTR_MASK |

Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>

Best regards,
Maxim Levitsky