linux-kernel - Re: [PATCH v3] x86/speculation/l1tf: Add KConfig for setting the L1D cache flush mode

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Message-ID: <5d2ccf3d-b473-cf30-b863-e29bb33b7284@redhat.com>
Date:   Sun, 5 Jul 2020 11:56:56 -0400
From:   Waiman Long <longman@...hat.com>
To:     Mike Rapoport <rppt@...ux.ibm.com>,
        Doug Anderson <dianders@...gle.com>
Cc:     Abhishek Bhardwaj <abhishekbh@...gle.com>,
        Anthony Steinhauser <asteinhauser@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Mark Gross <mgross@...ux.intel.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Tony Luck <tony.luck@...el.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>, kvm@...r.kernel.org,
        x86 <x86@...nel.org>
Subject: Re: [PATCH v3] x86/speculation/l1tf: Add KConfig for setting the L1D
 cache flush mode

On 7/5/20 11:23 AM, Mike Rapoport wrote:
>> Nothing prevents people from continuing to use the command line
>> options if they want, right?  This just allows a different default.
>> So if a distro is security focused and decided that it wanted a slower
>> / more secure default then it could ship that way but individual users
>> could still override, right?
> Well, nothing prevents you from continuing to use the command line as
> well;-)
>
> I can see why whould you want an ability to select compile time default
> for an option, but I'm really not thrilled by the added ifdefery.
>   

It turns out that CONFIG_KVM_VMENTRY_L1D_FLUSH values match the enum 
vmx_l1d_flush_state values. So one way to reduce the ifdefery is to do, 
for example,

+#ifdef CONFIG_KVM_VMENTRY_L1D_FLUSH
+#define VMENTER_L1D_FLUSH_DEFAULT CONFIG_KVM_VMENTRY_L1D_FLUSH
+#else
+#define VMENTER_L1D_FLUSH_DEFAULT      VMENTER_L1D_FLUSH_AUTO
#endif
-enum vmx_l1d_flush_state l1tf_vmx_mitigation = VMENTER_L1D_FLUSH_AUTO;
+enum vmx_l1d_flush_state l1tf_vmx_mitigation = VMENTER_L1D_FLUSH_DEFAULT;

Of course, we may need to add a comment on enum vmx_l1d_flush_state 
definition to highlight the dependency of CONFIG_KVM_VMENTRY_L1D_FLUSH 
on it to avoid future mismatch.

Cheers,
Longman