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

[Abhishek Bhardwaj <abhishekbh@google.com>]

On Sun, Jul 5, 2020 at 11:48 AM Waiman Long <longman@redhat.com> wrote:
>
> On 7/5/20 2:22 PM, Abhishek Bhardwaj wrote:
> > On Sun, Jul 5, 2020 at 8:57 AM Waiman Long <longman@redhat.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.

I'd rather avoid this dependency. However, if people are okay with the
CONFIG option then I am happy to oblige with whatever people agree on.
Can a maintainer chime in ? Waiman if you're the final authority on
this, will you accept the patch if I incorporated your suggestion ?

>
> Cheers,
> Longman
>


-- 
Abhishek