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

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On 7/5/20 2:22 PM, Abhishek Bhardwaj wrote:
On Sun, Jul 5, 2020 at 8:57 AM Waiman Long <longman@xxxxxxxxxx> 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




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