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

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Message-ID: <20190925205128.GB13637@redhat.com>
Date:   Wed, 25 Sep 2019 16:51:28 -0400
From:   Andrea Arcangeli <aarcange@...hat.com>
To:     Christophe de Dinechin <christophe.de.dinechin@...il.com>
Cc:     Vitaly Kuznetsov <vkuznets@...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 Wed, Sep 25, 2019 at 01:03:32PM +0200, Christophe de Dinechin wrote:
> 
> 
> > On 23 Sep 2019, at 11:31, Vitaly Kuznetsov <vkuznets@...hat.com> wrote:
> > 
> > Andrea Arcangeli <aarcange@...hat.com <mailto: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()?
> 
> Not sure, but if you do that, won’t the compiler generate a table and
> bring you back to square one? Or is there a reason why the mitigation
> is not needed for tables and indirect branches generated from switch
> statements?

When the kernel is built with retpolines the compiler is forbidden to
use a table for any switch. I pointed out the relevant commit earlier
in this thread. Instead the compiler will still try to bisect the
exit_reason trying to make the cost more equal for all exit_reason and
to reduce the number of checks, but we know the most likely exits so
it should be better to prioritize the most frequent exit reasons.

Thanks,
Andrea