linux-kernel - Re: [PATCH 3/7] kvm: vmx: pass MSR_IA32_SPEC_CTRL and MSR_IA32_PRED

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Date:   Tue, 9 Jan 2018 09:35:46 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Liran Alon <liran.alon@...cle.com>
Cc:     jmattson@...gle.com, bp@...en8.de, thomas.lendacky@....com,
        aliguori@...zon.com, linux-kernel@...r.kernel.org,
        dwmw2@...radead.org, kvm@...r.kernel.org
Subject: Re: [PATCH 3/7] kvm: vmx: pass MSR_IA32_SPEC_CTRL and
 MSR_IA32_PRED_CMD down to the guest

On 09/01/2018 00:58, Liran Alon wrote:
> 
> ----- pbonzini@...hat.com wrote:
> 
>> ----- Original Message -----
>>> From: "David Woodhouse" <dwmw2@...radead.org>
>>> To: "Paolo Bonzini" <pbonzini@...hat.com>,
>> linux-kernel@...r.kernel.org, kvm@...r.kernel.org
>>> Cc: jmattson@...gle.com, aliguori@...zon.com, "thomas lendacky"
>> <thomas.lendacky@....com>, bp@...en8.de
>>> Sent: Monday, January 8, 2018 8:41:07 PM
>>> Subject: Re: [PATCH 3/7] kvm: vmx: pass MSR_IA32_SPEC_CTRL and
>> MSR_IA32_PRED_CMD down to the guest
>>>
>>> On Mon, 2018-01-08 at 19:08 +0100, Paolo Bonzini wrote:
>>>>
>>>> +       if (have_spec_ctrl && vmx->spec_ctrl != 0)
>>>> +               wrmsrl(MSR_IA32_SPEC_CTRL, vmx->spec_ctrl);
>>>> +
>>>
>>> I think this one probably *is* safe even without an 'else lfence',
>>> which means that the CPU can speculate around it, but it wants a
>>> comment explaining that someone has properly analysed it and saying
>>> precisely why.
>>
>> This one is okay as long as there are no indirect jumps until
>> vmresume.  But the one on vmexit is only okay because right now
>> it's *disabling* IBRS.  Once IBRS is used by Linux, we'll need an
>> lfence there.  I'll add a comment.
>>
>> Paolo
> 
> That is true but from what I understand, there is an indirect branch from this point until vmresume.
> That indirect branch resides in atomic_switch_perf_msrs() immediately called after this WRMSR:
> atomic_switch_perf_msrs() -> perf_guest_get_msrs() -> x86_pmu.guest_get_msrs().

Sure, it has to move later as pointed out by other reviewers.

Paolo