Subject: Re: [PATCH 33/44] KVM: x86: Do VMX/SVM support checks directly in vendor code

From: Sean Christopherson <[email-masked]>
Date: Thu, 3 Nov 2022 18:35:05 +0000

On Thu, Nov 03, 2022, Paolo Bonzini wrote:
> On 11/3/22 00:19, Sean Christopherson wrote:
> > +	if (!boot_cpu_has(X86_FEATURE_MSR_IA32_FEAT_CTL) ||
> > +	    !boot_cpu_has(X86_FEATURE_VMX)) {
> > +		pr_err("VMX not enabled in MSR_IA32_FEAT_CTL\n");
> > +		return false;
> 
> I think the reference to the BIOS should remain in these messages and in
> svm.c (even though these days it's much less common for vendors to default
> to disabled virtualization in the system setup).

Ya, I'll figure out a way to mention BIOS/firmware.

> The check for X86_FEATURE_MSR_IA32_FEAT_CTL is not needed because
> init_ia32_feat_ctl() will clear X86_FEATURE_VMX if the rdmsr fail (and not
> set X86_FEATURE_MSR_IA32_FEAT_CTL).

It's technically required.  IA32_FEAT_CTL and thus KVM_INTEL depends on any of
CPU_SUP_{INTEL,CENATUR,ZHAOXIN}, but init_ia32_feat_ctl() is invoked if and only
if the actual CPU type matches one of the aforementioned CPU_SUP_*.

E.g. running a kernel built with

  CONFIG_CPU_SUP_INTEL=y
  CONFIG_CPU_SUP_AMD=y
  # CONFIG_CPU_SUP_HYGON is not set
  # CONFIG_CPU_SUP_CENTAUR is not set
  # CONFIG_CPU_SUP_ZHAOXIN is not set

on a Cenatur or Zhaoxin CPU will leave X86_FEATURE_VMX set but not set
X86_FEATURE_MSR_IA32_FEAT_CTL.  If VMX isn't enabled in MSR_IA32_FEAT_CTL, KVM
will get unexpected #UDs when trying to enable VMX.