linux-kernel - Re: [PATCH 6/7] x86/svm: Set IBPB when running a different VCPU

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Message-ID: <74e86dd8-804e-c9f2-098f-773283ac7065@redhat.com>
Date:   Tue, 9 Jan 2018 17:17:40 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Arjan van de Ven <arjan@...ux.intel.com>,
        Liran Alon <liran.alon@...cle.com>
Cc:     jmattson@...gle.com, dwmw@...zon.co.uk, bp@...en8.de,
        aliguori@...zon.com, thomas.lendacky@....com,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH 6/7] x86/svm: Set IBPB when running a different VCPU

On 09/01/2018 16:19, Arjan van de Ven wrote:
> On 1/9/2018 7:00 AM, Liran Alon wrote:
>>
>> ----- arjan@...ux.intel.com wrote:
>>
>>> On 1/9/2018 3:41 AM, Paolo Bonzini wrote:
>>>> The above ("IBRS simply disables the indirect branch predictor") was my
>>>> take-away message from private discussion with Intel.  My guess is that
>>>> the vendors are just handwaving a spec that doesn't match what they have
>>>> implemented, because honestly a microcode update is unlikely to do much
>>>> more than an old-fashioned chicken bit.  Maybe on Skylake it does
>>>> though, since the performance characteristics of IBRS are so different
>>>> from previous processors.  Let's ask Arjan who might have more
>>>> information about it, and hope he actually can disclose it...
>>>
>>> IBRS will ensure that, when set after the ring transition, no earlier
>>> branch prediction data is used for indirect branches while IBRS is
>>> set

Let me ask you my questions, which are independent of L0/L1/L2 terminology.

1) Is vmentry/vmexit considered a ring transition, even if the guest is
running in ring 0?  If IBRS=1 in the guest and the host is using IBRS,
the host will not do a wrmsr on exit.  Is this safe for the host kernel?

2) How will the future processors work where IBRS should always be =1?
Will they still need IBPB?  If I get a vmexit from a guest with IBRS=1,
and do a vmentry to the same VMCS *but with a different VPID*, will the
code running after the vmentry share BTB entries with code running
before the vmexit?

Thanks,

Paolo