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Subject: Re: [PATCH v2] x86/speculation/l1tf: Add KConfig for setting the L1D
 cache flush mode
To:     Abhishek Bhardwaj <abhishekbh@google.com>,
        LKML <linux-kernel@vger.kernel.org>
Cc:     Anthony Steinhauser <asteinhauser@google.com>,
        Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>, Ingo Molnar <mingo@redhat.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Mark Gross <mgross@linux.intel.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Pawan Gupta <pawan.kumar.gupta@linux.intel.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Sean Christopherson <sean.j.christopherson@intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Tony Luck <tony.luck@intel.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Waiman Long <longman@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>, kvm@vger.kernel.org,
        x86@kernel.org
References: <20200702213807.2511503-1-abhishekbh@google.com>
From:   Randy Dunlap <rdunlap@infradead.org>
Message-ID: <9c648d11-6b52-c755-d0a6-58f035ccd99d@infradead.org>
Date:   Thu, 2 Jul 2020 15:01:07 -0700
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Hi--

On 7/2/20 2:38 PM, Abhishek Bhardwaj wrote:
> This change adds a new kernel configuration that sets the l1d cache
> flush setting at compile time rather than at run time.
> 
> Signed-off-by: Abhishek Bhardwaj <abhishekbh@google.com>
> 
> ---
> 
> Changes in v2:
> - Fix typo in the help of the new KConfig.
> 
>  arch/x86/kernel/cpu/bugs.c |  8 ++++++++
>  arch/x86/kvm/Kconfig       | 17 +++++++++++++++++
>  2 files changed, 25 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..d375dcedd447d 100644
> --- a/arch/x86/kvm/Kconfig
> +++ b/arch/x86/kvm/Kconfig
> @@ -107,4 +107,21 @@ 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 && X86_64

Why does this apply only to KVM?

and the "X86 && X86_64" is more than is needed. Just "X86_64" alone
should be enough.


> +	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.
> +
> +# OK, it's a little counter-intuitive to do this, but it puts it neatly under
> +# the virtualization menu.
> +source "drivers/vhost/Kconfig"


I don't quite understand why this 'source' line is here.
Can you explain more about that, please?

It puts "VHOST drivers" in the menu 2 times, in 2 totally unrelated
places.  Seems like it could be confusing.

> +
>  endif # VIRTUALIZATION


-- 
~Randy