linux-kernel - Re: [PATCH 2/2] KVM: VMX: Drop unnecessary vmx_fb_clear_ctrl

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Message-ID: <217febbf-07fd-32dd-de6b-2e767de6a9eb@intel.com>
Date:   Thu, 8 Jun 2023 11:15:00 +0800
From:   Xiaoyao Li <xiaoyao.li@...el.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Chao Gao <chao.gao@...el.com>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
Subject: Re: [PATCH 2/2] KVM: VMX: Drop unnecessary
 vmx_fb_clear_ctrl_available "cache"

On 6/7/2023 8:43 AM, Sean Christopherson wrote:
> Now that KVM snapshots the host's MSR_IA32_ARCH_CAPABILITIES, drop the
> similar snapshot/cache of whether or not KVM is allowed to manipulate
> ARCH_CAPABILITIES.FB_CLEAR_CTRL.  The motivation for the cache was
> presumably to avoid the RDMSR, e.g. boot_cpu_has_bug() is quite cheap, and
> modifying the vCPU's MSR_IA32_ARCH_CAPABILITIES is an infrequent option
> and a relatively slow path.

Reviewed-by: Xiaoyao Li <xiaoyao.li@...el.com>

> Cc: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>   arch/x86/kvm/vmx/vmx.c | 17 +++--------------
>   1 file changed, 3 insertions(+), 14 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 42d1148f933c..17003660138a 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -237,9 +237,6 @@ static const struct {
>   #define L1D_CACHE_ORDER 4
>   static void *vmx_l1d_flush_pages;
>   
> -/* Control for disabling CPU Fill buffer clear */
> -static bool __read_mostly vmx_fb_clear_ctrl_available;
> -
>   static int vmx_setup_l1d_flush(enum vmx_l1d_flush_state l1tf)
>   {
>   	struct page *page;
> @@ -366,14 +363,6 @@ static int vmentry_l1d_flush_get(char *s, const struct kernel_param *kp)
>   	return sprintf(s, "%s\n", vmentry_l1d_param[l1tf_vmx_mitigation].option);
>   }
>   
> -static void vmx_setup_fb_clear_ctrl(void)
> -{
> -	if ((host_arch_capabilities & ARCH_CAP_FB_CLEAR_CTRL) &&
> -	    !boot_cpu_has_bug(X86_BUG_MDS) &&
> -	    !boot_cpu_has_bug(X86_BUG_TAA))
> -		vmx_fb_clear_ctrl_available = true;
> -}
> -
>   static __always_inline void vmx_disable_fb_clear(struct vcpu_vmx *vmx)
>   {
>   	u64 msr;
> @@ -399,7 +388,9 @@ static __always_inline void vmx_enable_fb_clear(struct vcpu_vmx *vmx)
>   
>   static void vmx_update_fb_clear_dis(struct kvm_vcpu *vcpu, struct vcpu_vmx *vmx)
>   {
> -	vmx->disable_fb_clear = vmx_fb_clear_ctrl_available;
> +	vmx->disable_fb_clear = (host_arch_capabilities & ARCH_CAP_FB_CLEAR_CTRL) &&
> +				!boot_cpu_has_bug(X86_BUG_MDS) &&
> +				!boot_cpu_has_bug(X86_BUG_TAA);
>   
>   	/*
>   	 * If guest will not execute VERW, there is no need to set FB_CLEAR_DIS
> @@ -8580,8 +8571,6 @@ static int __init vmx_init(void)
>   	if (r)
>   		goto err_l1d_flush;
>   
> -	vmx_setup_fb_clear_ctrl();
> -
>   	for_each_possible_cpu(cpu) {
>   		INIT_LIST_HEAD(&per_cpu(loaded_vmcss_on_cpu, cpu));
>