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Date:   Thu, 3 Nov 2022 18:35:05 +0000
From:   Sean Christopherson <seanjc@google.com>
To:     Paolo Bonzini <pbonzini@redhat.com>
Cc:     Marc Zyngier <maz@kernel.org>, Huacai Chen <chenhuacai@kernel.org>,
        Aleksandar Markovic <aleksandar.qemu.devel@gmail.com>,
        Anup Patel <anup@brainfault.org>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
        Christian Borntraeger <borntraeger@linux.ibm.com>,
        Janosch Frank <frankja@linux.ibm.com>,
        Claudio Imbrenda <imbrenda@linux.ibm.com>,
        Matthew Rosato <mjrosato@linux.ibm.com>,
        Eric Farman <farman@linux.ibm.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        James Morse <james.morse@arm.com>,
        Alexandru Elisei <alexandru.elisei@arm.com>,
        Suzuki K Poulose <suzuki.poulose@arm.com>,
        Oliver Upton <oliver.upton@linux.dev>,
        Atish Patra <atishp@atishpatra.org>,
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        Chao Gao <chao.gao@intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Yuan Yao <yuan.yao@intel.com>
Subject: Re: [PATCH 33/44] KVM: x86: Do VMX/SVM support checks directly in
 vendor code
Message-ID: <Y2QJ2TuyZImbFFvi@google.com>
References: <20221102231911.3107438-1-seanjc@google.com>
 <20221102231911.3107438-34-seanjc@google.com>
 <bfa98587-3b36-3834-a4b9-585a0e0aa56a@redhat.com>
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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.