Re: [PATCH v2 14/18] KVM: SVM: Check that the current CPU supports SVM in kvm_is_svm_supported()

[Huang, Kai]

On Fri, 2023-03-10 at 13:42 -0800, Sean Christopherson wrote:
> Check "this" CPU instead of the boot CPU when querying SVM support so that
> the per-CPU checks done during hardware enabling actually function as
> intended, i.e. will detect issues where SVM isn't support on all CPUs.
> 
> Disable migration for the use from svm_init() mostly so that the standard
> accessors for the per-CPU data can be used without getting yelled at by
> CONFIG_DEBUG_PREEMPT=y sanity checks.  Preventing the "disabled by BIOS"
> error message from reporting the wrong CPU is largely a bonus, as ensuring
> a stable CPU during module load is a non-goal for KVM.
> 
> Link: https://lore.kernel.org/all/ZAdxNgv0M6P63odE@xxxxxxxxxx
> Cc: Kai Huang <kai.huang@xxxxxxxxx>
> Cc: Chao Gao <chao.gao@xxxxxxxxx>
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>

Should we add:

Fixes: c82a5c5c53c5 ("KVM: x86: Do compatibility checks when onlining CPU")

As that commit introduced using raw_smp_processor_id() to get CPU id in
kvm_is_svm_supported() and print the CPU id out in error message?

> ---
>  arch/x86/kvm/svm/svm.c | 25 +++++++++++++++++++------
>  1 file changed, 19 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index 2934f185960d..f04b61c3d9d8 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -520,18 +520,20 @@ static void svm_init_osvw(struct kvm_vcpu *vcpu)
>  		vcpu->arch.osvw.status |= 1;
>  }
>  
> -static bool kvm_is_svm_supported(void)
> +static bool __kvm_is_svm_supported(void)
>  {
> -	int cpu = raw_smp_processor_id();
> +	int cpu = smp_processor_id();

Since we have made sure __kvm_is_svm_supported() is always performed on a stable
cpu, should we keep using raw_smp_processor_id()?  

It is faster than smp_processor_id() when CONFIG_DEBUG_PREEMPT=y, but yes the
latter can help to catch bug.

> +	struct cpuinfo_x86 *c = &cpu_data(cpu);
> +
>  	u64 vm_cr;
>  
> -	if (boot_cpu_data.x86_vendor != X86_VENDOR_AMD &&
> -	    boot_cpu_data.x86_vendor != X86_VENDOR_HYGON) {
> +	if (c->x86_vendor != X86_VENDOR_AMD &&
> +	    c->x86_vendor != X86_VENDOR_HYGON) {
>  		pr_err("CPU %d isn't AMD or Hygon\n", cpu);
>  		return false;
>  	}
>  
> -	if (!boot_cpu_has(X86_FEATURE_SVM)) {
> +	if (!cpu_has(c, X86_FEATURE_SVM)) {
>  		pr_err("SVM not supported by CPU %d\n", cpu);
>  		return false;
>  	}
> @@ -550,9 +552,20 @@ static bool kvm_is_svm_supported(void)
>  	return true;
>  }
>  
> +static bool kvm_is_svm_supported(void)
> +{
> +	bool supported;
> +
> +	migrate_disable();
> +	supported = __kvm_is_svm_supported();
> +	migrate_enable();
> +
> +	return supported;
> +}
> +
>  static int svm_check_processor_compat(void)
>  {
> -	if (!kvm_is_svm_supported())
> +	if (!__kvm_is_svm_supported())
>  		return -EIO;
>  
>  	return 0;
> -- 
> 2.40.0.rc1.284.g88254d51c5-goog
>