linux-kernel - Re: [PATCH] KVM: VMX: replace move_msr

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Date:   Sat, 4 Nov 2017 13:29:06 -0400 (EDT)
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     "Gustavo A. R. Silva" <garsilva@...eddedor.com>
Cc:     Radim Krčmář <rkrcmar@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: VMX: replace move_msr_up with swap macro



----- Original Message -----
> From: "Gustavo A. R. Silva" <garsilva@...eddedor.com>
> To: "Paolo Bonzini" <pbonzini@...hat.com>, "Radim Krčmář" <rkrcmar@...hat.com>, "Thomas Gleixner"
> <tglx@...utronix.de>, "Ingo Molnar" <mingo@...hat.com>, "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org
> Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org, "Gustavo A. R. Silva" <garsilva@...eddedor.com>
> Sent: Friday, November 3, 2017 11:58:19 PM
> Subject: [PATCH] KVM: VMX: replace move_msr_up with swap macro
> 
> Function move_msr_up is used to _manually_ swap MSR entries in MSR array.
> This function can be removed and replaced using the swap macro instead.
> 
> This code was detected with the help of Coccinelle.

I think move_msr_up should instead change into a function like

   void mark_msr_for_save(struct vcpu_vmx *vmx, int index)
   {
       swap(vmx->guest_msrs[index], vmx->guest_msrs[vmx->save_nmsrs]);
       vmx->save_nmsrs++;
   }

Using swap is useful, but it is also hiding what's going on exactly
(in addition, using ++ inside a macro argument might be calling for
trouble).

Paolo

> 
> Signed-off-by: Gustavo A. R. Silva <garsilva@...eddedor.com>
> ---
> The new lines are over 80 characters, but I think in this case that is
> preferable over splitting them.
> 
>  arch/x86/kvm/vmx.c | 24 ++++++------------------
>  1 file changed, 6 insertions(+), 18 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index e6c8ffa..210e491 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -2544,18 +2544,6 @@ static bool vmx_invpcid_supported(void)
>  	return cpu_has_vmx_invpcid() && enable_ept;
>  }
>  
> -/*
> - * Swap MSR entry in host/guest MSR entry array.
> - */
> -static void move_msr_up(struct vcpu_vmx *vmx, int from, int to)
> -{
> -	struct shared_msr_entry tmp;
> -
> -	tmp = vmx->guest_msrs[to];
> -	vmx->guest_msrs[to] = vmx->guest_msrs[from];
> -	vmx->guest_msrs[from] = tmp;
> -}
> -
>  static void vmx_set_msr_bitmap(struct kvm_vcpu *vcpu)
>  {
>  	unsigned long *msr_bitmap;
> @@ -2600,28 +2588,28 @@ static void setup_msrs(struct vcpu_vmx *vmx)
>  	if (is_long_mode(&vmx->vcpu)) {
>  		index = __find_msr_index(vmx, MSR_SYSCALL_MASK);
>  		if (index >= 0)
> -			move_msr_up(vmx, index, save_nmsrs++);
> +			swap(vmx->guest_msrs[index], vmx->guest_msrs[save_nmsrs++]);
>  		index = __find_msr_index(vmx, MSR_LSTAR);
>  		if (index >= 0)
> -			move_msr_up(vmx, index, save_nmsrs++);
> +			swap(vmx->guest_msrs[index], vmx->guest_msrs[save_nmsrs++]);
>  		index = __find_msr_index(vmx, MSR_CSTAR);
>  		if (index >= 0)
> -			move_msr_up(vmx, index, save_nmsrs++);
> +			swap(vmx->guest_msrs[index], vmx->guest_msrs[save_nmsrs++]);
>  		index = __find_msr_index(vmx, MSR_TSC_AUX);
>  		if (index >= 0 && guest_cpuid_has(&vmx->vcpu, X86_FEATURE_RDTSCP))
> -			move_msr_up(vmx, index, save_nmsrs++);
> +			swap(vmx->guest_msrs[index], vmx->guest_msrs[save_nmsrs++]);
>  		/*
>  		 * MSR_STAR is only needed on long mode guests, and only
>  		 * if efer.sce is enabled.
>  		 */
>  		index = __find_msr_index(vmx, MSR_STAR);
>  		if ((index >= 0) && (vmx->vcpu.arch.efer & EFER_SCE))
> -			move_msr_up(vmx, index, save_nmsrs++);
> +			swap(vmx->guest_msrs[index], vmx->guest_msrs[save_nmsrs++]);
>  	}
>  #endif
>  	index = __find_msr_index(vmx, MSR_EFER);
>  	if (index >= 0 && update_transition_efer(vmx, index))
> -		move_msr_up(vmx, index, save_nmsrs++);
> +		swap(vmx->guest_msrs[index], vmx->guest_msrs[save_nmsrs++]);
>  
>  	vmx->save_nmsrs = save_nmsrs;


>  
> --
> 2.7.4
> 
>