Re: [PATCH v3 15/18] KVM: VMX: Ensure CPU is stable when probing basic VMX support

Huang, Kai
2023-05-23T00:08:08Z

On Fri, 2023-05-12 at 16:50 -0700, Sean Christopherson wrote:
> Disable migration when probing VMX support during module load to ensure
> the CPU is stable, mostly to match similar SVM logic, where allowing
> migration effective requires deliberately writing buggy code. As a bonus,
> KVM won't report the wrong CPU to userspace if VMX is unsupported, but in
> practice that is a very, very minor bonus as the only way that reporting
> the wrong CPU would actually matter is if hardware is broken or if the
> system is misconfigured, i.e. if KVM gets migrated from a CPU that _does_
> support VMX to a CPU that does _not_ support VMX.
>
> Signed-off-by: Sean Christopherson <seanjc@google.com>

Reviewed-by: Kai Huang <kai.huang@intel.com>

> ---
> arch/x86/kvm/vmx/vmx.c | 17 ++++++++++++++---
> 1 file changed, 14 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index e00dba166a9e..008914396180 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -2740,9 +2740,9 @@ static int setup_vmcs_config(struct vmcs_config *vmcs_conf,
> return 0;
> }
>
> -static bool kvm_is_vmx_supported(void)
> +static bool __kvm_is_vmx_supported(void)
> {
> - int cpu = raw_smp_processor_id();
> + int cpu = smp_processor_id();
>
> if (!(cpuid_ecx(1) & feature_bit(VMX))) {
> pr_err("VMX not supported by CPU %d\n", cpu);
> @@ -2758,13 +2758,24 @@ static bool kvm_is_vmx_supported(void)
> return true;
> }
>
> +static bool kvm_is_vmx_supported(void)
> +{
> + bool supported;
> +
> + migrate_disable();
> + supported = __kvm_is_vmx_supported();
> + migrate_enable();
> +
> + return supported;
> +}
> +
> static int vmx_check_processor_compat(void)
> {
> int cpu = raw_smp_processor_id();
> struct vmcs_config vmcs_conf;
> struct vmx_capability vmx_cap;
>
> - if (!kvm_is_vmx_supported())
> + if (!__kvm_is_vmx_supported())
> return -EIO;
>
> if (setup_vmcs_config(&vmcs_conf, &vmx_cap) < 0) {
> --
> 2.40.1.606.ga4b1b128d6-goog
>