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 09:37:46 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     Andrea Arcangeli <aarcange@...hat.com>,
        Paolo Bonzini <pbonzini@...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

On Mon, Sep 23, 2019 at 11:31:58AM +0200, Vitaly Kuznetsov wrote:
> Andrea Arcangeli <aarcange@...hat.com> writes:
> 
> > 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
> > +		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
> >  		return kvm_vmx_exit_handlers[exit_reason](vcpu);
> 
> I agree with the identified set of most common vmexits, however, this
> still looks a bit random. Would it be too much if we get rid of
> kvm_vmx_exit_handlers completely replacing this code with one switch()?

Hmm, that'd require redirects for nVMX functions since they are set at
runtime.  That isn't necessarily a bad thing.  The approach could also be
used if Paolo's idea of making kvm_vmx_max_exit_handlers const allows the
compiler to avoid retpoline.

E.g.:

static int handle_vmx_instruction(struct kvm_vcpu *vcpu)
{
	if (nested)
		return nested_vmx_handle_exit(vcpu);

	kvm_queue_exception(vcpu, UD_VECTOR);
	return 1;
}