linux-kernel - Re: [PATCH 11/13] KVM: retpolines: x86: eliminate retpoline from vmx.c exit handlers

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Message-ID: <9a7d3134-b8eb-dfae-9c26-c21d3c1a1ea8@redhat.com>
Date:   Tue, 5 Nov 2019 11:20:42 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Andrea Arcangeli <aarcange@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>
Subject: Re: [PATCH 11/13] KVM: retpolines: x86: eliminate retpoline from
 vmx.c exit handlers

On 04/11/19 23:59, Andrea Arcangeli wrote:
> It's enough to check the exit value and issue a direct call to avoid
> the retpoline for all the common vmexit reasons.
> 
> Of course CONFIG_RETPOLINE already forbids gcc to use indirect jumps
> while compiling all switch() statements, however switch() would still
> allow the compiler to bisect the case value. It's more efficient to
> prioritize the most frequent vmexits instead.
> 
> The halt may be slow paths from the point of the guest, but not
> necessarily so from the point of the host if the host runs at full CPU
> capacity and no host CPU is ever left idle.
> 
> Signed-off-by: Andrea Arcangeli <aarcange@...hat.com>
> ---
>  arch/x86/kvm/vmx/vmx.c | 18 ++++++++++++++++--
>  1 file changed, 16 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index a6afa5f4a01c..582f837dc8c2 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -5905,9 +5905,23 @@ int kvm_x86_handle_exit(struct kvm_vcpu *vcpu)
>  	}
>  
>  	if (exit_reason < kvm_vmx_max_exit_handlers
> -	    && kvm_vmx_exit_handlers[exit_reason])
> +	    && kvm_vmx_exit_handlers[exit_reason]) {
> +#ifdef CONFIG_RETPOLINE
> +		if (exit_reason == EXIT_REASON_MSR_WRITE)
> +			return kvm_emulate_wrmsr(vcpu);
> +		else if (exit_reason == EXIT_REASON_PREEMPTION_TIMER)
> +			return handle_preemption_timer(vcpu);
> +		else if (exit_reason == EXIT_REASON_PENDING_INTERRUPT)
> +			return handle_interrupt_window(vcpu);
> +		else if (exit_reason == EXIT_REASON_EXTERNAL_INTERRUPT)
> +			return handle_external_interrupt(vcpu);
> +		else if (exit_reason == EXIT_REASON_HLT)
> +			return kvm_emulate_halt(vcpu);
> +		else if (exit_reason == EXIT_REASON_EPT_MISCONFIG)
> +			return handle_ept_misconfig(vcpu);
> +#endif
>  		return kvm_vmx_exit_handlers[exit_reason](vcpu);
> -	else {
> +	} else {
>  		vcpu_unimpl(vcpu, "vmx: unexpected exit reason 0x%x\n",
>  				exit_reason);
>  		dump_vmcs();
> 

Queued, thanks.

Paolo