linux-kernel - Re: [PATCH 3/8] kvm: vmx: pass MSR_IA32_SPEC_CTRL and MSR_IA32_PRED

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Message-ID: <CALMp9eThy8ZpzPJjWRVikmZxUdgbP7Km0tOBOXo20bxNuKxdQw@mail.gmail.com>
Date:   Fri, 12 Jan 2018 15:17:37 -0800
From:   Jim Mattson <jmattson@...gle.com>
To:     Nadav Amit <nadav.amit@...il.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Liran Alon <liran.alon@...cle.com>,
        "the arch/x86 maintainers" <x86@...nel.org>, dwmw@...zon.co.uk,
        Borislav Petkov <bp@...en8.de>,
        Anthony Liguori <aliguori@...zon.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        kvm list <kvm@...r.kernel.org>
Subject: Re: [PATCH 3/8] kvm: vmx: pass MSR_IA32_SPEC_CTRL and
 MSR_IA32_PRED_CMD down to the guest

Nadav,

See section 2.5.1.2 (paragraph 3) in
https://software.intel.com/sites/default/files/managed/c5/63/336996-Speculative-Execution-Side-Channel-Mitigations.pdf.

On Tue, Jan 9, 2018 at 9:03 PM, Nadav Amit <nadav.amit@...il.com> wrote:
> Paolo Bonzini <pbonzini@...hat.com> wrote:
>
>> On 09/01/2018 17:48, Liran Alon wrote:
>>>>> +  if (have_spec_ctrl) {
>>>>> +          rdmsrl(MSR_IA32_SPEC_CTRL, vmx->spec_ctrl);
>>>>> +          if (vmx->spec_ctrl != 0)
>>>>> +                  wrmsrl(MSR_IA32_SPEC_CTRL, 0);
>>>
>>> As I said also on the AMD patch, I think this is a bug.
>>> Intel specify that we should set IBRS bit even if it was already set on every #VMExit.
>>
>> That's correct (though I'd like to understand _why_---I'm not inclined
>> to blindly trust a spec), but for now it's saving a wrmsr of 0.  That is
>> quite obviously okay, and will be also okay after the bare-metal IBRS
>> patches.
>>
>> Of course the code will become something like
>>
>>       if (using_ibrs || vmx->spec_ctrl != 0)
>>               wrmsrl(MSR_IA32_SPEC_CTRL, host_ibrs);
>>
>> optimizing the case where the host is using retpolines.
>
> Excuse my ignorance: Can you point me to the specifications that mention “we
> should set IBRS bit even if it was already set on every #VMExit” ?
>
> Thanks,
> Nadav