linux-kernel - Re: [PATCH v3] x86/speculation, KVM: only IBPB for switch_mm_always

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Date:   Fri, 6 May 2022 15:42:00 +0000
From:   Jon Kohler <jon@...anix.com>
To:     Borislav Petkov <bp@...en8.de>
CC:     Jon Kohler <jon@...anix.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Balbir Singh <sblbir@...zon.com>,
        Kim Phillips <kim.phillips@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Kees Cook <keescook@...omium.org>,
        Waiman Long <longman@...hat.com>
Subject: Re: [PATCH v3] x86/speculation, KVM: only IBPB for
 switch_mm_always_ibpb on vCPU load



> On Apr 30, 2022, at 12:08 PM, Borislav Petkov <bp@...en8.de> wrote:
> 
> On Sat, Apr 30, 2022 at 02:50:35PM +0000, Jon Kohler wrote:
>> This is 100% a fair ask, I appreciate the diligence, as we’ve all been there
>> on the ‘other side’ of changes to complex areas and spend hours digging on
>> git history, LKML threads, SDM/APM, and other sources trying to derive
>> why the heck something is the way it is.
> 
> Yap, that's basically proving my point and why I want stuff to be
> properly documented so that the question "why was it done this way" can
> always be answered satisfactorily.
> 
>> AFAIK, the KVM IBPB is avoided when switching in between vCPUs
>> belonging to the same vmcs/vmcb (i.e. the same guest), e.g. you could 
>> have one VM highly oversubscribed to the host and you wouldn’t see
>> either the KVM IBPB or the switch_mm IBPB. All good. 
>> 
>> Reference vmx_vcpu_load_vmcs() and svm_vcpu_load() and the 
>> conditionals prior to the barrier.
> 
> So this is where something's still missing.
> 
>> However, the pain ramps up when you have a bunch of separate guests,
>> especially with a small amount of vCPUs per guest, so the switching is more
>> likely to be in between completely separate guests.
> 
> If the guests are completely separate, then it should fall into the
> switch_mm() case.
> 
> Unless it has something to do with, as I looked at the SVM side of
> things, the VMCBs:
> 
> 	if (sd->current_vmcb != svm->vmcb) {
> 
> So it is not only different guests but also within the same guest and
> when the VMCB of the vCPU is not the current one.
> 
> But then if VMCB of the vCPU is not the current, per-CPU VMCB, then that
> CPU ran another guest so in order for that other guest to attack the
> current guest, then its branch pred should be flushed.
> 
> But I'm likely missing a virt aspect here so I'd let Sean explain what
> the rules are...

Hey Sean - touching back on this one to see if were able to get some
thoughts together on this one?

Thanks - Jon

> 
> Thx.
> 
> -- 
> Regards/Gruss,
>    Boris.
> 
> https://urldefense.proofpoint.com/v2/url?u=https-3A__people.kernel.org_tglx_notes-2Dabout-2Dnetiquette&d=DwIDaQ&c=s883GpUCOChKOHiocYtGcg&r=NGPRGGo37mQiSXgHKm5rCQ&m=g8L6xyV5FDaMT1tJZ0GSAFqRAfzycwb-KqVGLeF9tuj2dl8To57JSPptw9_QKhn2&s=DU4mnTrLXbmOItsB0lTJNN4bgP2YC1n1Y2WeysN8PhM&e=