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

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Date:   Tue, 15 May 2018 14:57:52 -0700
From:   Jim Mattson <jmattson@...gle.com>
To:     Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Cc:     Wanpeng Li <kernellwp@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        kvm list <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>,
        Andrew Honig <ahonig@...gle.com>,
        Lars Bull <larsbull@...gle.com>
Subject: Re: [PATCH 3/3] KVM: VMX: Allow I/O port 0x80 bypass when userspace prefer

This does seem to allow a DoS from userspace if userspace prefers it.
That doesn't seem wise.

On Fri, May 11, 2018 at 8:44 AM, Konrad Rzeszutek Wilk
<konrad.wilk@...cle.com> wrote:
> 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
>>