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

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Message-ID: <48f82669-f1ff-0f5e-e531-ebbd151205f9@redhat.com>
Date:   Sun, 5 Jul 2020 14:48:01 -0400
From:   Waiman Long <longman@...hat.com>
To:     Abhishek Bhardwaj <abhishekbh@...gle.com>
Cc:     Mike Rapoport <rppt@...ux.ibm.com>,
        Doug Anderson <dianders@...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 2:22 PM, Abhishek Bhardwaj wrote:
> On Sun, Jul 5, 2020 at 8:57 AM Waiman Long <longman@...hat.com> wrote:
>> 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.
> I explicitly wanted to avoid doing that for this very reason. In my
> opinion this is brittle and bound to be missed
> sooner or later.
>
That is why I said a comment will have to be added to highlight this 
dependency. For instance,

+/*
+ * Three of the enums are explicitly assigned as the KVM_VMENTRY_L1D_FLUSH
+ * config entry in arch/x86/kvm/Kconfig depends on these values.
+ */
  enum vmx_l1d_flush_state {
         VMENTER_L1D_FLUSH_AUTO,
-       VMENTER_L1D_FLUSH_NEVER,
-       VMENTER_L1D_FLUSH_COND,
-       VMENTER_L1D_FLUSH_ALWAYS,
+       VMENTER_L1D_FLUSH_NEVER = 1,
+       VMENTER_L1D_FLUSH_COND = 2,
+       VMENTER_L1D_FLUSH_ALWAYS = 3,
         VMENTER_L1D_FLUSH_EPT_DISABLED,
         VMENTER_L1D_FLUSH_NOT_REQUIRED,
  };

Of course, this is just a suggestion.

Cheers,
Longman