linux-kernel - Re: [PATCH 4/8] kvm: vmx: Set IBPB when running a different VCPU

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Message-ID: <CALMp9eS4Y7+n+S49eC4n-0LazrmREpUATnELjEn2gRZQPeQirg@mail.gmail.com>
Date:   Fri, 12 Jan 2018 09:03:01 -0800
From:   Jim Mattson <jmattson@...gle.com>
To:     Wanpeng Li <kernellwp@...il.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>, kvm <kvm@...r.kernel.org>,
        Radim Krcmar <rkrcmar@...hat.com>,
        Liran Alon <liran.alon@...cle.com>,
        Anthony Liguori <aliguori@...zon.com>,
        Tom Lendacky <thomas.lendacky@....com>, dwmw@...zon.co.uk,
        Borislav Petkov <bp@...en8.de>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>
Subject: Re: [PATCH 4/8] kvm: vmx: Set IBPB when running a different VCPU

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@...il.com> wrote:
> 2018-01-09 20:03 GMT+08:00 Paolo Bonzini <pbonzini@...hat.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