linux-kernel - Re: [PATCH RFC v1 01/10] KVM: VMX: Move CPU_BASED_CR8_{LOAD,STORE}_EXITING filtering out of setup_vmcs

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Message-ID: <YrUAwPJTrYNT+zIt@google.com>
Date:   Fri, 24 Jun 2022 00:09:36 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Anirudh Rayabharam <anrayabh@...ux.microsoft.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Maxim Levitsky <mlevitsk@...hat.com>,
        linux-hyperv@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC v1 01/10] KVM: VMX: Move
 CPU_BASED_CR8_{LOAD,STORE}_EXITING filtering out of setup_vmcs_config()

On Wed, Jun 22, 2022, Vitaly Kuznetsov wrote:
> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
> ---
>  arch/x86/kvm/vmx/vmx.c | 11 ++++++-----
>  1 file changed, 6 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 5e14e4c40007..24da9e93bdab 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -2490,11 +2490,6 @@ static __init int setup_vmcs_config(struct vmcs_config *vmcs_conf,
>  	if (adjust_vmx_controls(min, opt, MSR_IA32_VMX_PROCBASED_CTLS,
>  				&_cpu_based_exec_control) < 0)
>  		return -EIO;
> -#ifdef CONFIG_X86_64
> -	if (_cpu_based_exec_control & CPU_BASED_TPR_SHADOW)
> -		_cpu_based_exec_control &= ~CPU_BASED_CR8_LOAD_EXITING &
> -					   ~CPU_BASED_CR8_STORE_EXITING;

Eww, who does a double "~" with an "&"?

> -#endif
>  	if (_cpu_based_exec_control & CPU_BASED_ACTIVATE_SECONDARY_CONTROLS) {
>  		min2 = 0;
>  		opt2 = SECONDARY_EXEC_VIRTUALIZE_APIC_ACCESSES |
> @@ -4285,6 +4280,12 @@ static u32 vmx_exec_control(struct vcpu_vmx *vmx)
>  {
>  	u32 exec_control = vmcs_config.cpu_based_exec_ctrl;
>  
> +#ifdef CONFIG_X86_64
> +	if (exec_control & CPU_BASED_TPR_SHADOW)
> +		exec_control &= ~CPU_BASED_CR8_LOAD_EXITING &
> +			~CPU_BASED_CR8_STORE_EXITING;

If you shove this done a few lines, then you can have a single set of #ifdefs,
and avoid restoring the controls a few lines later if it turns out KVM isn't
enabling the TPR shadow, e.g. (with fixup to use the more canonical ~(x | y)
pattern).

	if (!cpu_need_tpr_shadow(&vmx->vcpu))
		exec_control &= ~CPU_BASED_TPR_SHADOW;

#ifdef CONFIG_X86_64
	if (exec_control & CPU_BASED_TPR_SHADOW)
		exec_control &= ~(CPU_BASED_CR8_LOAD_EXITING |
				  CPU_BASED_CR8_STORE_EXITING);
	else
		exec_control |= CPU_BASED_CR8_STORE_EXITING |
				CPU_BASED_CR8_LOAD_EXITING;
#endif