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

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Message-Id: <5B82FF2C-0309-4D67-85E2-646AFB77B2FD@oracle.com>
Date:   Fri, 8 Nov 2019 17:52:19 +0200
From:   Liran Alon <liran.alon@...cle.com>
To:     Christophe de Dinechin <christophe.de.dinechin@...il.com>
Cc:     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 8 Nov 2019, at 17:35, Christophe de Dinechin <christophe.de.dinechin@...il.com> wrote:
> 
> 
> 
>> 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

Honestly, I don’t think any sane cloud provider will schedule vCPUs of different guests on same physical CPU core and report this to guest.
Therefore, I think this is only relevant for use-cases where the guest owner is also the host/hypervisor owner. And therefore, doesn’t need this
information exposed in a CPUID bit.
I see your point regarding how in theory it could be used, but I think we should wait and see if such use-case exists before defining this interface.

-Liran