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

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Date:   Mon, 16 Apr 2018 22:46:02 -0700
From:   Wanpeng Li <kernellwp@...il.com>
To:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Tim Shearer <tshearer@...aoptical.com>,
        Liran Alon <liran.alon@...cle.com>
Subject: [PATCH 3/3] KVM: VMX: Allow I/O port 0x80 bypass when userspace prefer

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.

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