linux-kernel - Re: [PATCH 2/3] KVM: X86: Provides userspace with a capability to not intercept HLT

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Date:   Thu, 8 Mar 2018 21:40:29 +0100
From:   Radim Krčmář <rkrcmar@...hat.com>
To:     Wanpeng Li <kernellwp@...il.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH 2/3] KVM: X86: Provides userspace with a capability to
 not intercept HLT

2018-03-01 17:49+0800, Wanpeng Li:
> From: Wanpeng Li <wanpengli@...cent.com>
> 
> If host CPUs are dedicated to a VM, we can avoid VM exits on HLT.
> This patch adds the per-VM non-HLT-exiting capability.
> 
> Cc: Paolo Bonzini <pbonzini@...hat.com>
> Cc: Radim Krčmář <rkrcmar@...hat.com>
> Signed-off-by: Wanpeng Li <wanpengli@...cent.com>
> ---
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index dff3a5d..fcf8339 100644
> --- a/arch/x86/kvm/svm.c
> +++ b/arch/x86/kvm/svm.c
> @@ -1394,6 +1394,9 @@ static void init_vmcb(struct vcpu_svm *svm)
>  		set_intercept(svm, INTERCEPT_MWAIT);
>  	}
>  
> +	if (!kvm_hlt_in_guest(svm->vcpu.kvm))
> +		set_intercept(svm, INTERCEPT_HLT);

We unconditionally set INTERCEPT_HLT just above, so that line has to be
removed.

> +
>  	control->iopm_base_pa = __sme_set(iopm_base);
>  	control->msrpm_base_pa = __sme_set(__pa(svm->msrpm));
>  	control->int_ctl = V_INTR_MASKING_MASK;
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> @@ -2525,6 +2525,19 @@ static int nested_vmx_check_exception(struct kvm_vcpu *vcpu, unsigned long *exit
>  	return 0;
>  }
>  
> +static void vmx_clear_hlt(struct kvm_vcpu *vcpu)
> +{
> +	/*
> +	 * Ensure that we clear the HLT state in the VMCS.  We don't need to
> +	 * explicitly skip the instruction because if the HLT state is set,
> +	 * then the instruction is already executing and RIP has already been
> +	 * advanced.
> +	 */
> +	if (kvm_hlt_in_guest(vcpu->kvm) &&
> +			vmcs_read32(GUEST_ACTIVITY_STATE) == GUEST_ACTIVITY_HLT)
> +		vmcs_write32(GUEST_ACTIVITY_STATE, GUEST_ACTIVITY_ACTIVE);
> +}

The clearing seems to be still missing around SMM -- I think you need to
call vmx_clear_hlt() from pre_enter_smm().

Thanks.