From: Wanpeng Li <>

Tim Shearer reported that "There is a guest which is running a packet 
forwarding app based on the DPDK ( 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 <>
Cc: Radim Krčmář <>
Cc: Tim Shearer <>
Cc: Liran Alon <>
Signed-off-by: Wanpeng Li <>
 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;

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