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SubjectRe: [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@tencent.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@redhat.com>
> Cc: Radim Krčmář <rkrcmar@redhat.com>
> Cc: Tim Shearer <tshearer@advaoptical.com>
> Cc: Liran Alon <liran.alon@oracle.com>
> Signed-off-by: Wanpeng Li <wanpengli@tencent.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
>

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