[PATCH v4] x86/speculation/l1tf: Add KConfig for setting the L1D cache flush mode

[PATCH v4] x86/speculation/l1tf: Add KConfig for setting the L1D cache flush mode

[Abhishek Bhardwaj]

This change adds a new kernel configuration that sets the l1d cache
flush setting at compile time rather than at run time.

The reasons for this change are as follows -

 - Kernel command line arguments are getting unwieldy. These parameters
 are not a scalable way to set the kernel config. They're intended as a
 super limited way for the bootloader to pass info to the kernel and
 also as a way for end users who are not compiling the kernel themselves
 to tweak the kernel behavior.

 - Also, if a user wants this setting from the start. It's a definite
 smell that it deserves to be a compile time thing rather than adding
 extra code plus whatever miniscule time at runtime to pass an
 extra argument.

 - Finally, it doesn't preclude the runtime / kernel command line way.
 Users are free to use those as well.

Signed-off-by: Abhishek Bhardwaj <abhishekbh@google.com>

---

Changes in v4:
- Add motivation for the change in the commit message.

Changes in v3:
- Change depends on to only x86_64.
- Remove copy paste errors at the end of the KConfig.

Changes in v2:
- Fix typo in the help of the new KConfig.

 arch/x86/kernel/cpu/bugs.c |  8 ++++++++
 arch/x86/kvm/Kconfig       | 13 +++++++++++++
 2 files changed, 21 insertions(+)

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 0b71970d2d3d2..1dcc875cf5547 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -1406,7 +1406,15 @@ enum l1tf_mitigations l1tf_mitigation __ro_after_init = L1TF_MITIGATION_FLUSH;
 #if IS_ENABLED(CONFIG_KVM_INTEL)
 EXPORT_SYMBOL_GPL(l1tf_mitigation);
 #endif
+#if (CONFIG_KVM_VMENTRY_L1D_FLUSH == 1)
+enum vmx_l1d_flush_state l1tf_vmx_mitigation = VMENTER_L1D_FLUSH_NEVER;
+#elif (CONFIG_KVM_VMENTRY_L1D_FLUSH == 2)
+enum vmx_l1d_flush_state l1tf_vmx_mitigation = VMENTER_L1D_FLUSH_COND;
+#elif (CONFIG_KVM_VMENTRY_L1D_FLUSH == 3)
+enum vmx_l1d_flush_state l1tf_vmx_mitigation = VMENTER_L1D_FLUSH_ALWAYS;
+#else
 enum vmx_l1d_flush_state l1tf_vmx_mitigation = VMENTER_L1D_FLUSH_AUTO;
+#endif
 EXPORT_SYMBOL_GPL(l1tf_vmx_mitigation);
 
 /*
diff --git a/arch/x86/kvm/Kconfig b/arch/x86/kvm/Kconfig
index b277a2db62676..1f85374a0b812 100644
--- a/arch/x86/kvm/Kconfig
+++ b/arch/x86/kvm/Kconfig
@@ -107,4 +107,17 @@ config KVM_MMU_AUDIT
 	 This option adds a R/W kVM module parameter 'mmu_audit', which allows
 	 auditing of KVM MMU events at runtime.
 
+config KVM_VMENTRY_L1D_FLUSH
+	int "L1D cache flush settings (1-3)"
+	range 1 3
+	default "2"
+	depends on KVM && X86_64
+	help
+	 This setting determines the L1D cache flush behavior before a VMENTER.
+	 This is similar to setting the option / parameter to
+	 kvm-intel.vmentry_l1d_flush.
+	 1 - Never flush.
+	 2 - Conditionally flush.
+	 3 - Always flush.
+
 endif # VIRTUALIZATION
-- 
2.27.0.383.g050319c2ae-goog

Re: [PATCH v4] x86/speculation/l1tf: Add KConfig for setting the L1D cache flush mode

[Randy Dunlap]

Hi again,

On 7/8/20 12:25 PM, Abhishek Bhardwaj wrote:
> diff --git a/arch/x86/kvm/Kconfig b/arch/x86/kvm/Kconfig
> index b277a2db62676..1f85374a0b812 100644
> --- a/arch/x86/kvm/Kconfig
> +++ b/arch/x86/kvm/Kconfig
> @@ -107,4 +107,17 @@ config KVM_MMU_AUDIT
>  	 This option adds a R/W kVM module parameter 'mmu_audit', which allows
>  	 auditing of KVM MMU events at runtime.
>  
> +config KVM_VMENTRY_L1D_FLUSH
> +	int "L1D cache flush settings (1-3)"
> +	range 1 3
> +	default "2"
> +	depends on KVM && X86_64
> +	help
> +	 This setting determines the L1D cache flush behavior before a VMENTER.
> +	 This is similar to setting the option / parameter to
> +	 kvm-intel.vmentry_l1d_flush.
> +	 1 - Never flush.
> +	 2 - Conditionally flush.
> +	 3 - Always flush.
> +
>  endif # VIRTUALIZATION

If you do a v5, the help text lines (under "help") should be indented
with one tab + 2 spaces according to Documentation/process/coding-style.rst.

-- 
~Randy

Re: [PATCH v4] x86/speculation/l1tf: Add KConfig for setting the L1D cache flush mode

[Abhishek Bhardwaj]

On Wed, Jul 8, 2020 at 12:34 PM Randy Dunlap <rdunlap@infradead.org> wrote:
>
> Hi again,
>
> On 7/8/20 12:25 PM, Abhishek Bhardwaj wrote:
> > diff --git a/arch/x86/kvm/Kconfig b/arch/x86/kvm/Kconfig
> > index b277a2db62676..1f85374a0b812 100644
> > --- a/arch/x86/kvm/Kconfig
> > +++ b/arch/x86/kvm/Kconfig
> > @@ -107,4 +107,17 @@ config KVM_MMU_AUDIT
> >        This option adds a R/W kVM module parameter 'mmu_audit', which allows
> >        auditing of KVM MMU events at runtime.
> >
> > +config KVM_VMENTRY_L1D_FLUSH
> > +     int "L1D cache flush settings (1-3)"
> > +     range 1 3
> > +     default "2"
> > +     depends on KVM && X86_64
> > +     help
> > +      This setting determines the L1D cache flush behavior before a VMENTER.
> > +      This is similar to setting the option / parameter to
> > +      kvm-intel.vmentry_l1d_flush.
> > +      1 - Never flush.
> > +      2 - Conditionally flush.
> > +      3 - Always flush.
> > +
> >  endif # VIRTUALIZATION
>
> If you do a v5, the help text lines (under "help") should be indented
> with one tab + 2 spaces according to Documentation/process/coding-style.rst.

Apologies for missing this. Fixed in v5 -
https://lkml.org/lkml/2020/7/8/1369

>
> --
> ~Randy
>


-- 
Abhishek