linux-kernel - Re: [PATCH 03/17] KVM: monolithic: x86: handle the request_immediate

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Date:   Mon, 23 Sep 2019 15:35:26 -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 03/17] KVM: monolithic: x86: handle the
 request_immediate_exit variation

On Fri, Sep 20, 2019 at 05:24:55PM -0400, Andrea Arcangeli wrote:
> request_immediate_exit is one of those few cases where the pointer to
> function of the method isn't fixed at build time and it requires
> special handling because hardware_setup() may override it at runtime.
> 
> Signed-off-by: Andrea Arcangeli <aarcange@...hat.com>
> ---
>  arch/x86/kvm/vmx/vmx_ops.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/vmx/vmx_ops.c b/arch/x86/kvm/vmx/vmx_ops.c
> index cdcad73935d9..25d441432901 100644
> --- a/arch/x86/kvm/vmx/vmx_ops.c
> +++ b/arch/x86/kvm/vmx/vmx_ops.c
> @@ -498,7 +498,10 @@ int kvm_x86_ops_check_nested_events(struct kvm_vcpu *vcpu, bool external_intr)
>  
>  void kvm_x86_ops_request_immediate_exit(struct kvm_vcpu *vcpu)
>  {
> -	vmx_request_immediate_exit(vcpu);
> +	if (likely(enable_preemption_timer))
> +		vmx_request_immediate_exit(vcpu);
> +	else
> +		__kvm_request_immediate_exit(vcpu);

Rather than wrap this in VMX code, what if we instead take advantage of a
monolithic module and add an inline to query enable_preemption_timer?
That'd likely save a few CALL/RET/JMP instructions and eliminate
__kvm_request_immediate_exit.

E.g. something like:

	if (req_immediate_exit) {
		kvm_make_request(KVM_REQ_EVENT, vcpu);
		if (kvm_x86_has_request_immediate_exit())
			kvm_x86_request_immediate_exit(vcpu);
		else
			smp_send_reschedule(vcpu->cpu);
	}

>  }
>  
>  void kvm_x86_ops_sched_in(struct kvm_vcpu *kvm, int cpu)