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Subject: Re: [PATCH] x86/bugs/multihit: Fix mitigation reporting when KVM is
 not in use
To:     Sean Christopherson <sean.j.christopherson@intel.com>
Cc:     Pawan Gupta <pawan.kumar.gupta@linux.intel.com>,
        Borislav Petkov <bp@alien8.de>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, x86@kernel.org,
        "H. Peter Anvin" <hpa@zytor.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>,
        Tony Luck <tony.luck@intel.com>,
        "Gomez Iglesias, Antonio" <antonio.gomez.iglesias@intel.com>,
        Andy Lutomirski <luto@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Fenghua Yu <fenghua.yu@intel.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Vincenzo Frascino <vincenzo.frascino@arm.com>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Anthony Steinhauser <asteinhauser@google.com>,
        Mike Rapoport <rppt@linux.ibm.com>,
        Mark Gross <mgross@linux.intel.com>,
        Waiman Long <longman@redhat.com>, linux-doc@vger.kernel.org,
        linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
        Jonathan Corbet <corbet@lwn.net>
References: <267631f4db4fd7e9f7ca789c2efaeab44103f68e.1594689154.git.pawan.kumar.gupta@linux.intel.com>
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From:   Dave Hansen <dave.hansen@intel.com>
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Message-ID: <807f553e-517e-dd3c-8c11-a07739367b0f@intel.com>
Date:   Wed, 15 Jul 2020 07:28:06 -0700
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On 7/14/20 5:51 PM, Sean Christopherson wrote:
> To do the above table, KVM will also need to update itlb_multihit_kvm_mitigation
> when it is unloaded, which seems rather silly.  That's partly why I suggested
> keying off CR4.VMXE as it doesn't require poking directly into KVM.  E.g. the
> entire fix becomes:

Failing to update itlb_multihit_kvm_mitigation leaves us with something
that's asymmetric.  A system with a never-loaded kvm module will say
something different than one that was loaded and then unloaded.

That's funky, but not the end of the world I guess.

> diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
> index ed54b3b21c39..4452df7f332d 100644
> --- a/arch/x86/kernel/cpu/bugs.c
> +++ b/arch/x86/kernel/cpu/bugs.c
> @@ -1447,7 +1447,12 @@ static ssize_t l1tf_show_state(char *buf)
> 
>  static ssize_t itlb_multihit_show_state(char *buf)
>  {
> -       if (itlb_multihit_kvm_mitigation)
> +       if (!boot_cpu_has(X86_FEATURE_MSR_IA32_FEAT_CTL) ||
> +           !boot_cpu_has(X86_FEATURE_VMX))
> +               return sprintf(buf, "KVM: Mitigation: VMX unsupported\n");
> +       else if (!(cr4_read_shadow() & X86_CR4_VMXE))
> +               return sprintf(buf, "KVM: Mitigation: VMX disabled\n");
> +       else if (itlb_multihit_kvm_mitigation)
>                 return sprintf(buf, "KVM: Mitigation: Split huge pages\n");
>         else
>                 return sprintf(buf, "KVM: Vulnerable\n");

That's at least short and sweet.  I wouldn't object to that at all.