linux-kernel - Re: [PATCH RFC] KVM: x86: tell guests if the exposed SMT topology is trustworthy

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Date:   Fri, 8 Nov 2019 16:35:17 +0100
From:   Christophe de Dinechin <christophe.de.dinechin@...il.com>
To:     Liran Alon <liran.alon@...cle.com>
Cc:     Christophe de Dinechin <christophe.de.dinechin@...il.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        KVM list <kvm@...r.kernel.org>, x86@...nel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Jim Mattson <jmattson@...gle.com>,
        open list <linux-kernel@...r.kernel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>
Subject: Re: [PATCH RFC] KVM: x86: tell guests if the exposed SMT topology is
 trustworthy



> On 7 Nov 2019, at 16:02, Liran Alon <liran.alon@...cle.com> wrote:
> 
> 
> 
>> On 7 Nov 2019, at 16:00, Christophe de Dinechin <christophe.de.dinechin@...il.com> wrote:
>> 
> 
>> 
>> I share that concern about the naming, although I do see some
>> value in exposing the cpu_smt_possible() result. I think it’s easier
>> to state that something does not work than to state something does
>> work.
>> 
>> Also, with respect to mitigation, we may want to split the two cases
>> that Paolo outlined, i.e. have KVM_HINTS_REALTIME,
>> KVM_HINTS_CORES_CROSSTALK and
>> KVM_HINTS_CORES_LEAKING,
>> where CORES_CROSSTALKS indicates there may be some
>> cross-talk between what the guest thinks are isolated cores,
>> and CORES_LEAKING indicates that cores may leak data
>> to some other guest.
>> 
>> The problem with my approach is that it is shouting “don’t trust me”
>> a bit too loudly.
> 
> I don’t see a value in exposing CORES_LEAKING to guest. As guest have nothing to do with it.

The guest could display / expose the information to guest sysadmins
and admin tools (e.g. through /proc).

While the kernel cannot mitigate, a higher-level product could for example
have a policy about which workloads can be deployed on a system which
may leak data to other VMs.

Christophe