linux-kernel - Re: [PATCH] KVM/VMX: Avoid CR3 VMEXIT during guest real mode when "unrestricted guest" is supported.

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Date:   Wed, 16 Aug 2017 11:25:02 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Lan Tianyu <tianyu.lan@...el.com>
Cc:     rkrcmar@...hat.com, tglx@...utronix.de, mingo@...hat.com,
        hpa@...or.com, x86@...nel.org, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM/VMX: Avoid CR3 VMEXIT during guest real mode when
 "unrestricted guest" is supported.

On 16/08/2017 03:58, Lan Tianyu wrote:
> These CR3 VMEXITs was introduced for platform without "unrestricted guest"
> support. This is to set ept identity table to guest CR3 in guest real
> mode because these platforms don't support ept real mode(CR0.PE and CR0.PG
> must be set to 1). But these VMEXITs is redundant for platforms with
> "unrestricted guest" support.

This is true, but I'm not sure it's a good idea to make things more
complex...  It is working code and is not a bottleneck anywhere.

Paolo

> Signed-off-by: Lan Tianyu <tianyu.lan@...el.com>
> ---
>  arch/x86/kvm/vmx.c | 22 +++++++++++++---------
>  1 file changed, 13 insertions(+), 9 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index 9b21b12..46dcf50 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -4221,18 +4221,20 @@ static void ept_update_paging_mode_cr0(unsigned long *hw_cr0,
>  		vmx_decache_cr3(vcpu);
>  	if (!(cr0 & X86_CR0_PG)) {
>  		/* From paging/starting to nonpaging */
> -		vmcs_write32(CPU_BASED_VM_EXEC_CONTROL,
> -			     vmcs_read32(CPU_BASED_VM_EXEC_CONTROL) |
> -			     (CPU_BASED_CR3_LOAD_EXITING |
> -			      CPU_BASED_CR3_STORE_EXITING));
> +		if (!enable_unrestricted_guest)
> +			vmcs_write32(CPU_BASED_VM_EXEC_CONTROL,
> +				     vmcs_read32(CPU_BASED_VM_EXEC_CONTROL) |
> +				     (CPU_BASED_CR3_LOAD_EXITING |
> +				      CPU_BASED_CR3_STORE_EXITING));
>  		vcpu->arch.cr0 = cr0;
>  		vmx_set_cr4(vcpu, kvm_read_cr4(vcpu));
>  	} else if (!is_paging(vcpu)) {
>  		/* From nonpaging to paging */
> -		vmcs_write32(CPU_BASED_VM_EXEC_CONTROL,
> -			     vmcs_read32(CPU_BASED_VM_EXEC_CONTROL) &
> -			     ~(CPU_BASED_CR3_LOAD_EXITING |
> -			       CPU_BASED_CR3_STORE_EXITING));
> +		if (!enable_unrestricted_guest)
> +			vmcs_write32(CPU_BASED_VM_EXEC_CONTROL,
> +				     vmcs_read32(CPU_BASED_VM_EXEC_CONTROL) &
> +				     ~(CPU_BASED_CR3_LOAD_EXITING |
> +				       CPU_BASED_CR3_STORE_EXITING));
>  		vcpu->arch.cr0 = cr0;
>  		vmx_set_cr4(vcpu, kvm_read_cr4(vcpu));
>  	}
> @@ -4311,7 +4313,9 @@ static void vmx_set_cr3(struct kvm_vcpu *vcpu, unsigned long cr3)
>  	}
>  
>  	vmx_flush_tlb(vcpu);
> -	vmcs_writel(GUEST_CR3, guest_cr3);
> +
> +	if (!enable_unrestricted_guest || !enable_ept)
> +		vmcs_writel(GUEST_CR3, guest_cr3);
>  }
>  
>  static int vmx_set_cr4(struct kvm_vcpu *vcpu, unsigned long cr4)
>