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References: <20180109120311.27565-1-pbonzini@redhat.com> <20180109120311.27565-5-pbonzini@redhat.com>
 <CANRm+Cwu+6TkA-dZaCJnMRY4PYMJ8JjQgegMPM8e6UqqJ+9Ohw@mail.gmail.com>
From: Jim Mattson <jmattson@google.com>
Date: Fri, 12 Jan 2018 09:03:01 -0800
Message-ID: <CALMp9eS4Y7+n+S49eC4n-0LazrmREpUATnELjEn2gRZQPeQirg@mail.gmail.com>
Subject: Re: [PATCH 4/8] kvm: vmx: Set IBPB when running a different VCPU
To: Wanpeng Li <kernellwp@gmail.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>,
        LKML <linux-kernel@vger.kernel.org>, kvm <kvm@vger.kernel.org>,
        Radim Krcmar <rkrcmar@redhat.com>,
        Liran Alon <liran.alon@oracle.com>,
        Anthony Liguori <aliguori@amazon.com>,
        Tom Lendacky <thomas.lendacky@amd.com>, dwmw@amazon.co.uk,
        Borislav Petkov <bp@alien8.de>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        Tim Chen <tim.c.chen@linux.intel.com>
Content-Type: text/plain; charset="UTF-8"
Sender: linux-kernel-owner@vger.kernel.org

The point behind the IPBP in vmx_vcpu_load is to prevent one VCPU from
steering the speculative execution of the next. If the VMCS address is
recycled, vmx_vcpu_load doesn't realize that the VCPUs are different,
and so it won't issue the IPBP.

On Thu, Jan 11, 2018 at 5:49 PM, Wanpeng Li <kernellwp@gmail.com> wrote:
> 2018-01-09 20:03 GMT+08:00 Paolo Bonzini <pbonzini@redhat.com>:
>>
>>         if (!already_loaded) {
>> @@ -4029,6 +4031,13 @@ static void free_loaded_vmcs(struct loaded_vmcs *loaded_vmcs)
>>         free_vmcs(loaded_vmcs->vmcs);
>>         loaded_vmcs->vmcs = NULL;
>>         WARN_ON(loaded_vmcs->shadow_vmcs != NULL);
>> +
>> +       /*
>> +        * The VMCS could be recycled, causing a false negative in
>> +        * vmx_vcpu_load; block speculative execution.
>> +        */
>> +       if (have_spec_ctrl)
>> +               wrmsrl(MSR_IA32_PRED_CMD, PRED_CMD_IBPB);
>>  }
>
> Intel guys told us the recycle is about the address of vmcs, not the
> content. Could you explain more why it matters?
>
> Regards,
> Wanpeng Li