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Date:   Tue, 10 May 2022 14:22:11 +0000
From:   Sean Christopherson <seanjc@google.com>
To:     Jon Kohler <jon@nutanix.com>
Cc:     Borislav Petkov <bp@alien8.de>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        "x86@kernel.org" <x86@kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Balbir Singh <sblbir@amazon.com>,
        Kim Phillips <kim.phillips@amd.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Kees Cook <keescook@chromium.org>,
        Waiman Long <longman@redhat.com>
Subject: Re: [PATCH v3] x86/speculation, KVM: only IBPB for
 switch_mm_always_ibpb on vCPU load
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On Sat, Apr 30, 2022, Jon Kohler wrote:
> 
> > On Apr 30, 2022, at 5:50 AM, Borislav Petkov <bp@alien8.de> wrote:
> > So let me try to understand this use case: you have a guest and a bunch
> > of vCPUs which belong to it. And that guest gets switched between those
> > vCPUs and KVM does IBPB flushes between those vCPUs.
> > 
> > So either I'm missing something - which is possible - but if not, that
> > "protection" doesn't make any sense - it is all within the same guest!
> > So that existing behavior was silly to begin with so we might just as
> > well kill it.
> 
> Close, its not 1 guest with a bunch of vCPU, its a bunch of guests with
> a small amount of vCPUs, thats the small nuance here, which is one of 
> the reasons why this was hard to see from the beginning. 
> 
> 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. 

No, KVM does not avoid IBPB when switching between vCPUs in a single VM.  Every
vCPU has a separate VMCS/VMCB, and so the scenario described above where a single
VM has a bunch of vCPUs running on a limited set of logical CPUs will emit IBPB
on every single switch.