Re: [PATCH 9/9] KVM: x86: Disable KVM_INTEL_PROVE_VE by default

[Paolo Bonzini]

On Sat, May 18, 2024 at 2:04 AM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
> Disable KVM's "prove #VE" support by default, as it provides no functional
> value, and even its sanity checking benefits are relatively limited.  I.e.
> it should be fully opt-in even on debug kernels, especially since EPT
> Violation #VE suppression appears to be buggy on some CPUs.

More #VE trapping than #VE suppression.

I wouldn't go so far as making it *depend* on DEBUG_KERNEL.  EXPERT
plus the scary help message is good enough.

What about this:

diff --git a/arch/x86/kvm/Kconfig b/arch/x86/kvm/Kconfig
index b6831e17ec31..2864608c7016 100644
--- a/arch/x86/kvm/Kconfig
+++ b/arch/x86/kvm/Kconfig
@@ -97,14 +97,15 @@ config KVM_INTEL

 config KVM_INTEL_PROVE_VE
         bool "Check that guests do not receive #VE exceptions"
-        depends on KVM_INTEL && DEBUG_KERNEL && EXPERT
+        depends on KVM_INTEL && EXPERT
         help
           Checks that KVM's page table management code will not incorrectly
           let guests receive a virtualization exception.  Virtualization
           exceptions will be trapped by the hypervisor rather than injected
           in the guest.

-          This should never be enabled in a production environment.
+          Note that #VE trapping appears to be buggy on some CPUs.
+          This should never be enabled in a production environment!

           If unsure, say N.


Paolo

> Opportunistically add a line in the help text to make it abundantly clear
> that KVM_INTEL_PROVE_VE should never be enabled in a production
> environment.
>
> Suggested-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
>  arch/x86/kvm/Kconfig | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/kvm/Kconfig b/arch/x86/kvm/Kconfig
> index 2a7f69abcac3..3468efc4be55 100644
> --- a/arch/x86/kvm/Kconfig
> +++ b/arch/x86/kvm/Kconfig
> @@ -97,15 +97,15 @@ config KVM_INTEL
>
>  config KVM_INTEL_PROVE_VE
>          bool "Check that guests do not receive #VE exceptions"
> -        default KVM_PROVE_MMU || DEBUG_KERNEL
> -        depends on KVM_INTEL
> +        depends on KVM_INTEL && DEBUG_KERNEL && EXPERT
>          help
> -
>            Checks that KVM's page table management code will not incorrectly
>            let guests receive a virtualization exception.  Virtualization
>            exceptions will be trapped by the hypervisor rather than injected
>            in the guest.
>
> +          This should never be enabled in a production environment.
> +
>            If unsure, say N.
>
>  config X86_SGX_KVM
> --
> 2.45.0.215.g3402c0e53f-goog
>