linux-kernel - Re: [RFC PATCH 1/5] KVM: vmx: speed up emulation of invalid guest state

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Date:	Wed, 16 Apr 2014 19:52:58 -0300
From:	Marcelo Tosatti <mtosatti@...hat.com>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [RFC PATCH 1/5] KVM: vmx: speed up emulation of invalid guest
 state

On Thu, Mar 27, 2014 at 12:30:34PM +0100, Paolo Bonzini wrote:
> About 25% of the time spent in emulation of invalid guest state
> is wasted in checking whether emulation is required for the next
> instruction.  However, this almost never changes except when a
> segment register (or TR or LDTR) changes, or when there is a mode
> transition (i.e. CR0 changes).
> 
> In fact, vmx_set_segment and vmx_set_cr0 already modify
> vmx->emulation_required (except that the former for some reason
> uses |= instead of just an assignment).  So there is no need to
> call guest_state_valid in the emulation loop.
> 
> Emulation performance test results indicate 530-870 cycles
> for common instructions, versus 740-1110 before this patch.
> 
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> ---
>  arch/x86/kvm/vmx.c | 5 ++---
>  1 file changed, 2 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index 1320e0f8e611..73aa522db47b 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -3629,7 +3629,7 @@ static void vmx_set_segment(struct kvm_vcpu *vcpu,
>  	vmcs_write32(sf->ar_bytes, vmx_segment_access_rights(var));
>  
>  out:
> -	vmx->emulation_required |= emulation_required(vcpu);
> +	vmx->emulation_required = emulation_required(vcpu);
>  }
>  
>  static void vmx_get_cs_db_l_bits(struct kvm_vcpu *vcpu, int *db, int *l)
> @@ -5580,7 +5580,7 @@ static int handle_invalid_guest_state(struct kvm_vcpu *vcpu)
>  	cpu_exec_ctrl = vmcs_read32(CPU_BASED_VM_EXEC_CONTROL);
>  	intr_window_requested = cpu_exec_ctrl & CPU_BASED_VIRTUAL_INTR_PENDING;
>  
> -	while (!guest_state_valid(vcpu) && count-- != 0) {
> +	while (vmx->emulation_required && count-- != 0) {
>  		if (intr_window_requested && vmx_interrupt_allowed(vcpu))
>  			return handle_interrupt_window(&vmx->vcpu);
>  
> @@ -5614,7 +5614,6 @@ static int handle_invalid_guest_state(struct kvm_vcpu *vcpu)
>  			schedule();
>  	}
>  
> -	vmx->emulation_required = emulation_required(vcpu);
>  out:
>  	return ret;
>  }
> -- 
> 1.8.3.1

How about handling VM-entry error due to invalid state with

vmx->emulation_required = true;
continue to main vcpu loop;



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