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

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Date:   Tue, 9 Jan 2018 07:33:27 -0800 (PST)
From:   Liran Alon <liran.alon@...cle.com>
To:     <arjan@...ux.intel.com>
Cc:     <jmattson@...gle.com>, <dwmw@...zon.co.uk>, <bp@...en8.de>,
        <thomas.lendacky@....com>, <aliguori@...zon.com>,
        <pbonzini@...hat.com>, <linux-kernel@...r.kernel.org>,
        <kvm@...r.kernel.org>
Subject: Re: [PATCH 6/7] x86/svm: Set IBPB when running a different VCPU


----- arjan@...ux.intel.com 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
> > 
> > Consider the following scenario:
> > 1. L1 runs with IBRS=1 in Ring0.
> > 2. L1 restores L2 SPEC_CTRL and enters into L2.
> > 3. L1 VMRUN exits into L0 which backups L1 SPEC_CTRL and enters L2
> (using same VMCB).
> > 4. L2 populates BTB/BHB with values and cause a hypercall which
> #VMExit into L0.
> > 5. L0 backups L2 SPEC_CTRL and writes IBRS=1.
> > 6. L0 restores L1 SPEC_CTRL and enters L1.
> > 7. L1 backups L2 SPEC_CTRL and writes IBRS=1.
> > 
> 
> I'm sorry I'm not familiar with your L0/L1/L2 terminology
> (maybe it's before coffee has had time to permeate the brain)

These are standard terminology for guest levels:
L0 == hypervisor that runs on bare-metal
L1 == hypervisor that runs as L0 guest.
L2 == software that runs as L1 guest.
(We are talking about nested virtualization here)

-Liran