linux-kernel - Re: [PATCH v4 1/7] KVM: VMX: Introduce generic fastpath handler

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Message-ID: <20200501141159.GC3798@linux.intel.com>
Date:   Fri, 1 May 2020 07:11:59 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     Wanpeng Li <kernellwp@...il.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Haiwei Li <lihaiwei@...cent.com>
Subject: Re: [PATCH v4 1/7] KVM: VMX: Introduce generic fastpath handler

On Thu, Apr 30, 2020 at 03:28:46PM +0200, Vitaly Kuznetsov wrote:
> Wanpeng Li <kernellwp@...il.com> writes:
> 
> > From: Wanpeng Li <wanpengli@...cent.com>
> >
> > Introduce generic fastpath handler to handle MSR fastpath, VMX-preemption
> > timer fastpath etc, move it after vmx_complete_interrupts() in order that
> > later patch can catch the case vmexit occurred while another event was
> > being delivered to guest. There is no obversed performance difference for
> > IPI fastpath testing after this move.
> >
> > Tested-by: Haiwei Li <lihaiwei@...cent.com>
> > Cc: Haiwei Li <lihaiwei@...cent.com>
> > Signed-off-by: Wanpeng Li <wanpengli@...cent.com>
> > ---
> >  arch/x86/kvm/vmx/vmx.c | 21 ++++++++++++++++-----
> >  1 file changed, 16 insertions(+), 5 deletions(-)
> >
> > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> > index 3ab6ca6..9b5adb4 100644
> > --- a/arch/x86/kvm/vmx/vmx.c
> > +++ b/arch/x86/kvm/vmx/vmx.c
> > @@ -6583,6 +6583,20 @@ void vmx_update_host_rsp(struct vcpu_vmx *vmx, unsigned long host_rsp)
> >  	}
> >  }
> >  
> > +static enum exit_fastpath_completion vmx_exit_handlers_fastpath(struct kvm_vcpu *vcpu)
> > +{
> > +	if (!is_guest_mode(vcpu)) {
> 
> Nitpick: do we actually expect to have any fastpath handlers anytime
> soon? If not, we could've written this as
> 
> 	if (is_guest_mode(vcpu))
> 		return EXIT_FASTPATH_NONE;
> 
> and save on identation)

Agreed.  An alternative approach would be to do the check in the caller, e.g.

	if (is_guest_mode(vcpu))
		return EXIT_FASTPATH_NONE;

	return vmx_exit_handlers_fastpath(vcpu);

I don't have a strong preference either way.

> > +		switch (to_vmx(vcpu)->exit_reason) {
> > +		case EXIT_REASON_MSR_WRITE:
> > +			return handle_fastpath_set_msr_irqoff(vcpu);
> > +		default:
> > +			return EXIT_FASTPATH_NONE;
> > +		}
> > +	}
> > +
> > +	return EXIT_FASTPATH_NONE;
> > +}
> > +
> >  bool __vmx_vcpu_run(struct vcpu_vmx *vmx, unsigned long *regs, bool launched);
> >  
> >  static enum exit_fastpath_completion vmx_vcpu_run(struct kvm_vcpu *vcpu)
> > @@ -6757,17 +6771,14 @@ static enum exit_fastpath_completion vmx_vcpu_run(struct kvm_vcpu *vcpu)
> >  	if (unlikely(vmx->exit_reason & VMX_EXIT_REASONS_FAILED_VMENTRY))
> >  		return EXIT_FASTPATH_NONE;
> >  
> > -	if (!is_guest_mode(vcpu) && vmx->exit_reason == EXIT_REASON_MSR_WRITE)
> > -		exit_fastpath = handle_fastpath_set_msr_irqoff(vcpu);
> > -	else
> > -		exit_fastpath = EXIT_FASTPATH_NONE;
> > -
> >  	vmx->loaded_vmcs->launched = 1;
> >  	vmx->idt_vectoring_info = vmcs_read32(IDT_VECTORING_INFO_FIELD);
> >  
> >  	vmx_recover_nmi_blocking(vmx);
> >  	vmx_complete_interrupts(vmx);
> >  
> > +	exit_fastpath = vmx_exit_handlers_fastpath(vcpu);

No need for capturing the result in a local variable, just return the function
call.

> > +
> >  	return exit_fastpath;
> >  }
> 
> Reviewed-by: Vitaly Kuznetsov <vkuznets@...hat.com>
> 
> -- 
> Vitaly
>