linux-kernel - Re: [PATCH v2 10/15] KVM: x86/vmx: Check Arch LBR config when return perf capabilities

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Message-ID: <Y9RKaEzZ9tSKW1ZB@google.com>
Date:   Fri, 27 Jan 2023 22:04:24 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Yang Weijiang <weijiang.yang@...el.com>
Cc:     pbonzini@...hat.com, jmattson@...gle.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, like.xu.linux@...il.com,
        kan.liang@...ux.intel.com, wei.w.wang@...el.com,
        Like Xu <like.xu@...ux.intel.com>
Subject: Re: [PATCH v2 10/15] KVM: x86/vmx: Check Arch LBR config when return
 perf capabilities

On Thu, Nov 24, 2022, Yang Weijiang wrote:
>  	return vmcs_config.cpu_based_2nd_exec_ctrl &
> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> index b28be793de29..59bdd9873fb5 100644
> --- a/arch/x86/kvm/vmx/nested.c
> +++ b/arch/x86/kvm/vmx/nested.c
> @@ -2360,6 +2360,10 @@ static void prepare_vmcs02_early(struct vcpu_vmx *vmx, struct loaded_vmcs *vmcs0
>  		if (guest_efer != host_efer)
>  			exec_control |= VM_ENTRY_LOAD_IA32_EFER;
>  	}
> +
> +	if (cpu_has_vmx_arch_lbr())
> +		exec_control &= ~VM_ENTRY_LOAD_IA32_LBR_CTL;
> +
>  	vm_entry_controls_set(vmx, exec_control);
>  
>  	/*
> @@ -2374,6 +2378,10 @@ static void prepare_vmcs02_early(struct vcpu_vmx *vmx, struct loaded_vmcs *vmcs0
>  		exec_control |= VM_EXIT_LOAD_IA32_EFER;
>  	else
>  		exec_control &= ~VM_EXIT_LOAD_IA32_EFER;
> +
> +	if (cpu_has_vmx_arch_lbr())
> +		exec_control &= ~VM_EXIT_CLEAR_IA32_LBR_CTL;

This is wrong.  If KVM doesn't enumerate suport to L1, then L1's value needs to
be preserved on entry/exit to/from L1<->L2.  I.e. just delete these lines.

>  	vm_exit_controls_set(vmx, exec_control);
>  
>  	/*
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 2ab4c33b5008..359da38a19a1 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -2599,6 +2599,7 @@ static __init int setup_vmcs_config(struct vmcs_config *vmcs_conf,
>  		{ VM_ENTRY_LOAD_IA32_EFER,		VM_EXIT_LOAD_IA32_EFER },
>  		{ VM_ENTRY_LOAD_BNDCFGS,		VM_EXIT_CLEAR_BNDCFGS },
>  		{ VM_ENTRY_LOAD_IA32_RTIT_CTL,		VM_EXIT_CLEAR_IA32_RTIT_CTL },
> +		{ VM_ENTRY_LOAD_IA32_LBR_CTL, 		VM_EXIT_CLEAR_IA32_LBR_CTL },
>  	};
>  
>  	memset(vmcs_conf, 0, sizeof(*vmcs_conf));
> @@ -4794,6 +4795,9 @@ static void vmx_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event)
>  	vpid_sync_context(vmx->vpid);
>  
>  	vmx_update_fb_clear_dis(vcpu, vmx);
> +
> +	if (!init_event && cpu_has_vmx_arch_lbr())
> +		vmcs_write64(GUEST_IA32_LBR_CTL, 0);

This belongs in init_vmcs().

>  }
>  
>  static void vmx_enable_irq_window(struct kvm_vcpu *vcpu)
> @@ -6191,6 +6195,10 @@ void dump_vmcs(struct kvm_vcpu *vcpu)
>  	    vmentry_ctl & VM_ENTRY_LOAD_IA32_PERF_GLOBAL_CTRL)
>  		pr_err("PerfGlobCtl = 0x%016llx\n",
>  		       vmcs_read64(GUEST_IA32_PERF_GLOBAL_CTRL));
> +	if (kvm_cpu_cap_has(X86_FEATURE_ARCH_LBR) &&

Follow every other field and check the VMCS support, not the cap.

> +	    vmentry_ctl & VM_ENTRY_LOAD_IA32_LBR_CTL)
> +		pr_err("ArchLBRCtl = 0x%016llx\n",
> +		       vmcs_read64(GUEST_IA32_LBR_CTL));
>  	if (vmentry_ctl & VM_ENTRY_LOAD_BNDCFGS)
>  		pr_err("BndCfgS = 0x%016llx\n", vmcs_read64(GUEST_BNDCFGS));
>  	pr_err("Interruptibility = %08x  ActivityState = %08x\n",

...

> diff --git a/arch/x86/kvm/vmx/vmx.h b/arch/x86/kvm/vmx/vmx.h
> index a3da84f4ea45..f68c8a53a248 100644
> --- a/arch/x86/kvm/vmx/vmx.h
> +++ b/arch/x86/kvm/vmx/vmx.h
> @@ -493,7 +493,8 @@ static inline u8 vmx_get_rvi(void)
>  	 VM_ENTRY_LOAD_IA32_EFER |					\
>  	 VM_ENTRY_LOAD_BNDCFGS |					\
>  	 VM_ENTRY_PT_CONCEAL_PIP |					\
> -	 VM_ENTRY_LOAD_IA32_RTIT_CTL)
> +	 VM_ENTRY_LOAD_IA32_RTIT_CTL |					\
> +	 VM_ENTRY_LOAD_IA32_LBR_CTL)
>  
>  #define __KVM_REQUIRED_VMX_VM_EXIT_CONTROLS				\
>  	(VM_EXIT_SAVE_DEBUG_CONTROLS |					\
> @@ -515,7 +516,8 @@ static inline u8 vmx_get_rvi(void)
>  	       VM_EXIT_LOAD_IA32_EFER |					\
>  	       VM_EXIT_CLEAR_BNDCFGS |					\
>  	       VM_EXIT_PT_CONCEAL_PIP |					\
> -	       VM_EXIT_CLEAR_IA32_RTIT_CTL)
> +	       VM_EXIT_CLEAR_IA32_RTIT_CTL |				\
> +	       VM_EXIT_CLEAR_IA32_LBR_CTL)

Enabling these flags by default is wrong.  KVM will clear LBR_CTL on VM-Exit when
arch LBRs are supported, and AFAICT, never restore the host's values.  And that
will happen regardless of whether or not the guest is using arch LBRs.  I assume
the correct approach is to toggle these fields, but I'll be damned if I can figure
out what the intended behavior of the existing LBR suport is.  I'll follow up in
the cover letter.