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Subject: Re: [PATCH] x86/bugs/multihit: Fix mitigation reporting when KVM is
 not in use
To:     Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Cc:     Sean Christopherson <sean.j.christopherson@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>
 <20200714014540.GH29725@linux.intel.com>
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From:   Dave Hansen <dave.hansen@intel.com>
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Message-ID: <ba442a51-294e-8624-9a69-5613ff050551@intel.com>
Date:   Tue, 14 Jul 2020 12:54:26 -0700
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On 7/14/20 12:17 PM, Pawan Gupta wrote:
> On Tue, Jul 14, 2020 at 07:57:53AM -0700, Dave Hansen wrote:
>> Let's stick to things which are at least static per reboot.  Checking
>> for X86_FEATURE_VMX or even CONFIG_KVM_INTEL seems like a good stopping
>> point.  "Could this kernel run a naughty guest?"  If so, report
>> "Vulnerable".  It's the same as Meltdown: "Could this kernel run
>> untrusted code?"  If so, report "Vulnerable".
> 
> Thanks, These are good inputs. So what I need to add is a boot time
> check for VMX feature and report "Vulnerable" or "Not
> affected(VMX disabled)".
> 
> Are you suggesting to not change the reporting when KVM deploys the
> "Split huge pages" mitigation? Is this because VMX can still be used by
> other VMMs?
> 
> The current mitigation reporting is very specific to KVM:
> 
> 	- "KVM: Vulnerable"
> 	- "KVM: Mitigation: Split huge pages"
> 
> As the kernel doesn't know about the mitigation state of out-of-tree
> VMMs can we add VMX reporting to always say vulnerable when VMX is
> enabled:
> 
> 	- "VMX: Vulnerable, KVM: Vulnerable"
> 	- "VMX: Vulnerable, KVM: Mitigation: Split huge pages"
> 
> And if VMX is disabled report:
> 
> 	- "VMX: Not affected(VMX disabled)"

I see three inputs and four possible states (sorry for the ugly table,
it was this or a spreadsheet :):

X86_FEATURE_VMX	CONFIG_KVM_*	hpage split  Result	   Reason
	N		x	    x	     Not Affected  No VMX
	Y		N	    x	     Not affected  No KVM
	Y		Y	    Y	     Mitigated	   hpage split
	Y		Y	    N	     Vulnerable

I don't think we should worry about out-of-tree VMX.