linux-kernel - Re: [PATCH] KVM: SVM: Connect 'npt' module param to KVM's internal 'npt

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Date:   Fri, 05 Mar 2021 10:54:39 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH] KVM: SVM: Connect 'npt' module param to KVM's internal
 'npt_enabled'

Sean Christopherson <seanjc@...gle.com> writes:

> Directly connect the 'npt' param to the 'npt_enabled' variable so that
> runtime adjustments to npt_enabled are reflected in sysfs.  Move the
> !PAE restriction to a runtime check to ensure NPT is forced off if the
> host is using 2-level paging, and add a comment explicitly stating why
> NPT requires a 64-bit kernel or a kernel with PAE enabled.
>
> Opportunistically switch the param to octal permissions.
>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>  arch/x86/kvm/svm/svm.c | 27 ++++++++++++++-------------
>  1 file changed, 14 insertions(+), 13 deletions(-)
>
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index 54610270f66a..0ee74321461e 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -115,13 +115,6 @@ static const struct svm_direct_access_msrs {
>  	{ .index = MSR_INVALID,				.always = false },
>  };
>  
> -/* enable NPT for AMD64 and X86 with PAE */
> -#if defined(CONFIG_X86_64) || defined(CONFIG_X86_PAE)
> -bool npt_enabled = true;
> -#else
> -bool npt_enabled;
> -#endif
> -
>  /*
>   * These 2 parameters are used to config the controls for Pause-Loop Exiting:
>   * pause_filter_count: On processors that support Pause filtering(indicated
> @@ -170,9 +163,12 @@ module_param(pause_filter_count_shrink, ushort, 0444);
>  static unsigned short pause_filter_count_max = KVM_SVM_DEFAULT_PLE_WINDOW_MAX;
>  module_param(pause_filter_count_max, ushort, 0444);
>  
> -/* allow nested paging (virtualized MMU) for all guests */
> -static int npt = true;
> -module_param(npt, int, S_IRUGO);
> +/*
> + * Use nested page tables by default.  Note, NPT may get forced off by
> + * svm_hardware_setup() if it's unsupported by hardware or the host kernel.
> + */
> +bool npt_enabled = true;
> +module_param_named(npt, npt_enabled, bool, 0444);
>  
>  /* allow nested virtualization in KVM/SVM */
>  static int nested = true;
> @@ -988,12 +984,17 @@ static __init int svm_hardware_setup(void)
>  			goto err;
>  	}
>  
> +	/*
> +	 * KVM's MMU doesn't support using 2-level paging for itself, and thus
> +	 * NPT isn't supported if the host is using 2-level paging since host
> +	 * CR4 is unchanged on VMRUN.
> +	 */
> +	if (!IS_ENABLED(CONFIG_X86_64) && !IS_ENABLED(CONFIG_X86_PAE))
> +		npt_enabled = false;
> +
>  	if (!boot_cpu_has(X86_FEATURE_NPT))
>  		npt_enabled = false;
>  
> -	if (npt_enabled && !npt)
> -		npt_enabled = false;
> -

Reviewed-by: Vitaly Kuznetsov <vkuznets@...hat.com>

>  	kvm_configure_mmu(npt_enabled, get_max_npt_level(), PG_LEVEL_1G);
>  	pr_info("kvm: Nested Paging %sabled\n", npt_enabled ? "en" : "dis");

(unrelated to your patch but) I'd suggest we demote this pr_info() to
pr_debug() or keep it but print the message only when NPT is disabled as
in the overwhelming majority of cases it is enabled. Also, we don't seem
to print EPT status when kvm-intel is loaded.

-- 
Vitaly