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

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Date:   Tue, 9 Jan 2018 07:00:38 -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>,
        <aliguori@...zon.com>, <thomas.lendacky@....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 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.

Just to make sure I understand:
You state that L2 BTB/BHB won't be used by L1 because L1 have set IBRS=1 in step (7).
And that is even though L1 & L2 could both be running in SVM guest-mode & Ring0 from physical CPU perspective. Therefore, having the same prediction-mode.

So basically you are saying that IBRS don't make sure to avoid using BTB/BHB from lower prediction-modes but instead just make sure to avoid usage of all BTB/BHB while IBRS is set.

Did I understand everything correctly?

Thanks,
-Liran

> 
> (this is a english summary of two pages of technical spec so it lacks
> the language lawyer precision)
> 
> because of this promise, the implementation tends to be impactful
> and it is very strongly recommended that retpoline is used instead of
> IBRS.
> (with all the caveats already on lkml)
> 
> the IBPB is different, this is a covenient thing for switching between
> VM guests etc