linux-kernel - Re: [PATCH 3/3] KVM: VMX: Allow I/O port 0x80 bypass when userspace prefer

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Message-ID: <20180511154458.GI27459@char.us.oracle.com>
Date:   Fri, 11 May 2018 11:44:58 -0400
From:   Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:     Wanpeng Li <kernellwp@...il.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Tim Shearer <tshearer@...aoptical.com>,
        Liran Alon <liran.alon@...cle.com>
Subject: Re: [PATCH 3/3] KVM: VMX: Allow I/O port 0x80 bypass when userspace
 prefer

On Mon, Apr 16, 2018 at 10:46:02PM -0700, Wanpeng Li wrote:
> From: Wanpeng Li <wanpengli@...cent.com>
> 
> Tim Shearer reported that "There is a guest which is running a packet 
> forwarding app based on the DPDK (dpdk.org). The packet receive routine 
> writes to 0xc070 using glibc's "outw_p" function which does an additional 
> write to I/O port 80. It does this write for every packet that's received, 
> causing a flood of KVM userspace context switches". He uses mpstat to 
> observe a CPU performing L2 packet forwarding on a pinned guest vCPU, 
> the guest time is 95 percent when allowing I/O port 0x80 bypass, however, 
> it is 65.78 percent when I/O port 0x80 bypss is disabled.  
> 
> This patch allows I/O port 0x80 bypass when userspace prefer.

s/prefer/requests it/
> 

Perhaps:

Reported-by: Tim Shearer as well?

> Cc: Paolo Bonzini <pbonzini@...hat.com>
> Cc: Radim Krčmář <rkrcmar@...hat.com>
> Cc: Tim Shearer <tshearer@...aoptical.com>
> Cc: Liran Alon <liran.alon@...cle.com>
> Signed-off-by: Wanpeng Li <wanpengli@...cent.com>
> ---
>  arch/x86/kvm/vmx.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index ebf1140..d3e5fef 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -10118,6 +10118,13 @@ static int vmx_vm_init(struct kvm *kvm)
>  			goto out;
>  		memset(kvm_vmx->vmx_io_bitmap[i], 0xff, PAGE_SIZE);
>  	}
> +	if (kvm->arch.ioport_disable_intercept)	{
> +		/*
> +		 * Allow direct access to the PC debug port (it is often used for I/O
> +		 * delays, but the vmexits simply slow things down).
> +		 */
> +		clear_bit(0x80, kvm_vmx->vmx_io_bitmap[VMX_IO_BITMAP_A]);
> +	}
>  	return 0;
>  
>  out:
> -- 
> 2.7.4
>