Re: [PATCH v2] KVM: vmx: update sec exec controls for UMIP iff emulating UMIP

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On 30/04/2018 19:01, Sean Christopherson wrote:
> Update SECONDARY_EXEC_DESC for UMIP emulation if and only UMIP
> is actually being emulated.  Skipping the VMCS update eliminates
> unnecessary VMREAD/VMWRITE when UMIP is supported in hardware,
> and on platforms that don't have SECONDARY_VM_EXEC_CONTROL.  The
> latter case resolves a bug where KVM would fill the kernel log
> with warnings due to failed VMWRITEs on older platforms.
> 
> Fixes: 0367f205a3b7 ("KVM: vmx: add support for emulating UMIP")
> Cc: stable@xxxxxxxxxxxxxxx #4.16
> Reported-by: Paolo Zeppegno <pzeppegno@xxxxxxxxx>
> Suggested-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> Suggested-by: Radim Krčmář <rkrcmar@xxxxxxxxxx>
> Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
> ---
> v2: Fix the bug simply by skipping VMCS updates when UMIP is not
>     being emulated, as suggested by Paolo and again by Radim.
>     The approach of updating the VMCS only when CR4.UMIP changed
>     was not guaranteed to work in all cases.  Optimizing away
>     VMREAD/VMWRITE will be tackled in a separate series.
> 
>  arch/x86/kvm/vmx.c | 28 +++++++++++++++-------------
>  1 file changed, 15 insertions(+), 13 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index aafcc9881e88..53d85439e5e5 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -1494,6 +1494,12 @@ static inline bool cpu_has_vmx_vmfunc(void)
>  		SECONDARY_EXEC_ENABLE_VMFUNC;
>  }
>  
> +static bool vmx_umip_emulated(void)
> +{
> +	return vmcs_config.cpu_based_2nd_exec_ctrl &
> +		SECONDARY_EXEC_DESC;
> +}
> +
>  static inline bool report_flexpriority(void)
>  {
>  	return flexpriority_enabled;
> @@ -4776,14 +4782,16 @@ static int vmx_set_cr4(struct kvm_vcpu *vcpu, unsigned long cr4)
>  	else
>  		hw_cr4 |= KVM_PMODE_VM_CR4_ALWAYS_ON;
>  
> -	if ((cr4 & X86_CR4_UMIP) && !boot_cpu_has(X86_FEATURE_UMIP)) {
> -		vmcs_set_bits(SECONDARY_VM_EXEC_CONTROL,
> -			      SECONDARY_EXEC_DESC);
> -		hw_cr4 &= ~X86_CR4_UMIP;
> -	} else if (!is_guest_mode(vcpu) ||
> -	           !nested_cpu_has2(get_vmcs12(vcpu), SECONDARY_EXEC_DESC))
> -		vmcs_clear_bits(SECONDARY_VM_EXEC_CONTROL,
> +	if (!boot_cpu_has(X86_FEATURE_UMIP) && vmx_umip_emulated()) {
> +		if (cr4 & X86_CR4_UMIP) {
> +			vmcs_set_bits(SECONDARY_VM_EXEC_CONTROL,
>  				SECONDARY_EXEC_DESC);
> +			hw_cr4 &= ~X86_CR4_UMIP;
> +		} else if (!is_guest_mode(vcpu) ||
> +			!nested_cpu_has2(get_vmcs12(vcpu), SECONDARY_EXEC_DESC))
> +			vmcs_clear_bits(SECONDARY_VM_EXEC_CONTROL,
> +					SECONDARY_EXEC_DESC);
> +	}
>  
>  	if (cr4 & X86_CR4_VMXE) {
>  		/*
> @@ -9512,12 +9520,6 @@ static bool vmx_xsaves_supported(void)
>  		SECONDARY_EXEC_XSAVES;
>  }
>  
> -static bool vmx_umip_emulated(void)
> -{
> -	return vmcs_config.cpu_based_2nd_exec_ctrl &
> -		SECONDARY_EXEC_DESC;
> -}
> -
>  static void vmx_recover_nmi_blocking(struct vcpu_vmx *vmx)
>  {
>  	u32 exit_intr_info;
> 

Queued, thanks.

Paolo



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