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

[Waiman Long]

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.
+ */
Âenum vmx_l1d_flush_state {
ÂÂÂÂÂÂÂ VMENTER_L1D_FLUSH_AUTO,
-ÂÂÂÂÂÂ VMENTER_L1D_FLUSH_NEVER,
-ÂÂÂÂÂÂ VMENTER_L1D_FLUSH_COND,
-ÂÂÂÂÂÂ VMENTER_L1D_FLUSH_ALWAYS,
+ÂÂÂÂÂÂ VMENTER_L1D_FLUSH_NEVER = 1,
+ÂÂÂÂÂÂ VMENTER_L1D_FLUSH_COND = 2,
+ÂÂÂÂÂÂ VMENTER_L1D_FLUSH_ALWAYS = 3,
ÂÂÂÂÂÂÂ VMENTER_L1D_FLUSH_EPT_DISABLED,
ÂÂÂÂÂÂÂ VMENTER_L1D_FLUSH_NOT_REQUIRED,
Â};

Of course, this is just a suggestion.

Cheers,
Longman