linux-kernel - Re: [PATCH] KVM: VMX: Invert the inlining of MSR interception helpers

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Message-ID: <febab7f6-aab8-6278-5782-07d75dd40166@redhat.com>
Date:   Mon, 26 Apr 2021 11:19:54 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: VMX: Invert the inlining of MSR interception helpers

On 24/04/21 00:19, Sean Christopherson wrote:
> Invert the inline declarations of the MSR interception helpers between
> the wrapper, vmx_set_intercept_for_msr(), and the core implementations,
> vmx_{dis,en}able_intercept_for_msr().  Letting the compiler _not_
> inline the implementation reduces KVM's code footprint by ~3k bytes.
> 
> Back when the helpers were added in commit 904e14fb7cb9 ("KVM: VMX: make
> MSR bitmaps per-VCPU"), both the wrapper and the implementations were
> __always_inline because the end code distilled down to a few conditionals
> and a bit operation.  Today, the implementations involve a variety of
> checks and bit ops in order to support userspace MSR filtering.
> 
> Furthermore, the vast majority of calls to manipulate MSR interception
> are not performance sensitive, e.g. vCPU creation and x2APIC toggling.
> On the other hand, the one path that is performance sensitive, dynamic
> LBR passthrough, uses the wrappers, i.e. is largely untouched by
> inverting the inlining.
> 
> In short, forcing the low level MSR interception code to be inlined no
> longer makes sense.
> 
> No functional change intended.
> 
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>   arch/x86/kvm/vmx/vmx.c | 17 ++---------------
>   arch/x86/kvm/vmx/vmx.h | 15 +++++++++++++--
>   2 files changed, 15 insertions(+), 17 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 6501d66167b8..b77bc72d97a4 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -362,8 +362,6 @@ static const struct kernel_param_ops vmentry_l1d_flush_ops = {
>   module_param_cb(vmentry_l1d_flush, &vmentry_l1d_flush_ops, NULL, 0644);
>   
>   static u32 vmx_segment_access_rights(struct kvm_segment *var);
> -static __always_inline void vmx_disable_intercept_for_msr(struct kvm_vcpu *vcpu,
> -							  u32 msr, int type);
>   
>   void vmx_vmexit(void);
>   
> @@ -3818,8 +3816,7 @@ static void vmx_set_msr_bitmap_write(ulong *msr_bitmap, u32 msr)
>   		__set_bit(msr & 0x1fff, msr_bitmap + 0xc00 / f);
>   }
>   
> -static __always_inline void vmx_disable_intercept_for_msr(struct kvm_vcpu *vcpu,
> -							  u32 msr, int type)
> +void vmx_disable_intercept_for_msr(struct kvm_vcpu *vcpu, u32 msr, int type)
>   {
>   	struct vcpu_vmx *vmx = to_vmx(vcpu);
>   	unsigned long *msr_bitmap = vmx->vmcs01.msr_bitmap;
> @@ -3864,8 +3861,7 @@ static __always_inline void vmx_disable_intercept_for_msr(struct kvm_vcpu *vcpu,
>   		vmx_clear_msr_bitmap_write(msr_bitmap, msr);
>   }
>   
> -static __always_inline void vmx_enable_intercept_for_msr(struct kvm_vcpu *vcpu,
> -							 u32 msr, int type)
> +void vmx_enable_intercept_for_msr(struct kvm_vcpu *vcpu, u32 msr, int type)
>   {
>   	struct vcpu_vmx *vmx = to_vmx(vcpu);
>   	unsigned long *msr_bitmap = vmx->vmcs01.msr_bitmap;
> @@ -3898,15 +3894,6 @@ static __always_inline void vmx_enable_intercept_for_msr(struct kvm_vcpu *vcpu,
>   		vmx_set_msr_bitmap_write(msr_bitmap, msr);
>   }
>   
> -void vmx_set_intercept_for_msr(struct kvm_vcpu *vcpu,
> -						      u32 msr, int type, bool value)
> -{
> -	if (value)
> -		vmx_enable_intercept_for_msr(vcpu, msr, type);
> -	else
> -		vmx_disable_intercept_for_msr(vcpu, msr, type);
> -}
> -
>   static u8 vmx_msr_bitmap_mode(struct kvm_vcpu *vcpu)
>   {
>   	u8 mode = 0;
> diff --git a/arch/x86/kvm/vmx/vmx.h b/arch/x86/kvm/vmx/vmx.h
> index 19fe09fad2fe..008cb87ff088 100644
> --- a/arch/x86/kvm/vmx/vmx.h
> +++ b/arch/x86/kvm/vmx/vmx.h
> @@ -392,8 +392,19 @@ void vmx_update_host_rsp(struct vcpu_vmx *vmx, unsigned long host_rsp);
>   bool __vmx_vcpu_run(struct vcpu_vmx *vmx, unsigned long *regs, bool launched);
>   int vmx_find_loadstore_msr_slot(struct vmx_msrs *m, u32 msr);
>   void vmx_ept_load_pdptrs(struct kvm_vcpu *vcpu);
> -void vmx_set_intercept_for_msr(struct kvm_vcpu *vcpu,
> -	u32 msr, int type, bool value);
> +
> +void vmx_disable_intercept_for_msr(struct kvm_vcpu *vcpu, u32 msr, int type);
> +void vmx_enable_intercept_for_msr(struct kvm_vcpu *vcpu, u32 msr, int type);
> +
> +static inline void vmx_set_intercept_for_msr(struct kvm_vcpu *vcpu, u32 msr,
> +					     int type, bool value)
> +{
> +	if (value)
> +		vmx_enable_intercept_for_msr(vcpu, msr, type);
> +	else
> +		vmx_disable_intercept_for_msr(vcpu, msr, type);
> +}
> +
>   void vmx_update_cpu_dirty_logging(struct kvm_vcpu *vcpu);
>   
>   static inline u8 vmx_get_rvi(void)
> 

Queued (and pretty much closing the 5.13 kvm/next branch after this).

Paolo