linux-kernel - Re: [PATCH 15/17] KVM: retpolines: x86: eliminate retpoline from vmx.c exit handlers

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Date:   Mon, 23 Sep 2019 09:28:37 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Andrea Arcangeli <aarcange@...hat.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        "Dr. David Alan Gilbert" <dgilbert@...hat.com>,
        Marcelo Tosatti <mtosatti@...hat.com>,
        Peter Xu <peterx@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 15/17] KVM: retpolines: x86: eliminate retpoline from
 vmx.c exit handlers

Subject should be something like:

  KVM: VMX: Make direct calls to fast path VM-Exit handlers

On Fri, Sep 20, 2019 at 05:25:07PM -0400, 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.
> 
> Signed-off-by: Andrea Arcangeli <aarcange@...hat.com>
> ---
>  arch/x86/kvm/vmx/vmx.c | 24 ++++++++++++++++++++++--
>  1 file changed, 22 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index a6e597025011..9aa73e216df2 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -5866,9 +5866,29 @@ static int vmx_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

I'd strongly prefer to make any optimization of this type unconditional,
i.e. not dependent on CONFIG_RETPOLINE.  Today, I'm comfortable testing
KVM only with CONFIG_RETPOLINE=y since the only KVM-specific difference
is additive code in vmx_vmexit.  That would no longer be the case if KVM
has non-trivial code differences for retpoline.

> +		if (exit_reason == EXIT_REASON_MSR_WRITE)
> +			return handle_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 handle_halt(vcpu);
> +		else if (exit_reason == EXIT_REASON_PAUSE_INSTRUCTION)
> +			return handle_pause(vcpu);
> +		else if (exit_reason == EXIT_REASON_MSR_READ)
> +			return handle_rdmsr(vcpu);
> +		else if (exit_reason == EXIT_REASON_CPUID)
> +			return handle_cpuid(vcpu);
> +		else if (exit_reason == EXIT_REASON_EPT_MISCONFIG)
> +			return handle_ept_misconfig(vcpu);
> +#endif

This can be hoisted above the if statement.

>  		return kvm_vmx_exit_handlers[exit_reason](vcpu);
> -	else {
> +	} else {
>  		vcpu_unimpl(vcpu, "vmx: unexpected exit reason 0x%x\n",
>  				exit_reason);
>  		dump_vmcs();