From: Liran Alon <liran.alon@oracle.com>
To: Christophe de Dinechin <christophe.de.dinechin@gmail.com>
Cc: Sean Christopherson <sean.j.christopherson@intel.com>,
Vitaly Kuznetsov <vkuznets@redhat.com>,
kvm@vger.kernel.org, x86@kernel.org,
Paolo Bonzini <pbonzini@redhat.com>,
Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
Jim Mattson <jmattson@google.com>,
linux-kernel@vger.kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
"Peter Zijlstra (Intel)" <peterz@infradead.org>
Subject: Re: [PATCH RFC] KVM: x86: tell guests if the exposed SMT topology is trustworthy
Date: Thu, 7 Nov 2019 17:02:26 +0200 [thread overview]
Message-ID: <713ECF67-6A6C-4956-8AC6-7F4C05961328@oracle.com> (raw)
In-Reply-To: <943488A8-2DD7-4471-B3C7-9F21A0B0BCF9@dinechin.org>
> On 7 Nov 2019, at 16:00, Christophe de Dinechin <christophe.de.dinechin@gmail.com> wrote:
>
>
>
>> On 6 Nov 2019, at 00:25, Sean Christopherson <sean.j.christopherson@intel.com> wrote:
>>
>> On Tue, Nov 05, 2019 at 11:37:50AM -0800, Sean Christopherson wrote:
>>> On Tue, Nov 05, 2019 at 05:17:37PM +0100, Vitaly Kuznetsov wrote:
>>>> Virtualized guests may pick a different strategy to mitigate hardware
>>>> vulnerabilities when it comes to hyper-threading: disable SMT completely,
>>>> use core scheduling, or, for example, opt in for STIBP. Making the
>>>> decision, however, requires an extra bit of information which is currently
>>>> missing: does the topology the guest see match hardware or if it is 'fake'
>>>> and two vCPUs which look like different cores from guest's perspective can
>>>> actually be scheduled on the same physical core. Disabling SMT or doing
>>>> core scheduling only makes sense when the topology is trustworthy.
>>>>
>>>> Add two feature bits to KVM: KVM_FEATURE_TRUSTWORTHY_SMT with the meaning
>>>> that KVM_HINTS_TRUSTWORTHY_SMT bit answers the question if the exposed SMT
>>>> topology is actually trustworthy. It would, of course, be possible to get
>>>> away with a single bit (e.g. 'KVM_FEATURE_FAKE_SMT') and not lose backwards
>>>> compatibility but the current approach looks more straightforward.
>>>
>>> I'd stay away from "trustworthy", especially if this is controlled by
>>> userspace. Whether or not the hint is trustworthy is purely up to the
>>> guest. Right now it doesn't really matter, but that will change as we
>>> start moving pieces of the host out of the guest's TCB.
>>>
>>> It may make sense to split the two (or even three?) cases, e.g.
>>> KVM_FEATURE_NO_SMT and KVM_FEATURE_ACCURATE_TOPOLOGY. KVM can easily
>>> enforce NO_SMT _today_, i.e. allow it to be set if and only if SMT is
>>> truly disabled. Verifying that the topology exposed to the guest is legit
>>> is a completely different beast.
>>
>> Scratch the ACCURATE_TOPOLOGY idea, I doubt there's a real use case for
>> setting ACCURATE_TOPOLOGY and not KVM_HINTS_REALTIME. A feature flag to
>> state that SMT is disabled seems simple and useful.
A bit such as NoNonArchitecturalCoreSharing can be set even when host SMT is enabled.
For example, when host use core-scheduling to group together vCPUs that run as sibling hyperthreads.
Therefore, I wouldn’t want to tie the feature-flag semantics to host SMT being enabled/disabled.
It’s just true that this bit can be set when host SMT is disabled.
>
> 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.
-Liran
next prev parent reply other threads:[~2019-11-07 15:03 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-11-05 16:17 [PATCH RFC] KVM: x86: tell guests if the exposed SMT topology is trustworthy Vitaly Kuznetsov
2019-11-05 17:17 ` Liran Alon
2019-11-05 17:30 ` Liran Alon
2019-11-05 17:35 ` Jim Mattson
2019-11-05 19:37 ` Sean Christopherson
2019-11-05 23:25 ` Sean Christopherson
2019-11-07 10:38 ` Vitaly Kuznetsov
[not found] ` <943488A8-2DD7-4471-B3C7-9F21A0B0BCF9@dinechin.org>
2019-11-07 15:02 ` Liran Alon [this message]
2019-11-08 15:35 ` Christophe de Dinechin
2019-11-08 15:52 ` Liran Alon
2019-11-05 20:02 ` Peter Zijlstra
2019-11-05 23:25 ` Sean Christopherson
2019-11-06 8:32 ` Peter Zijlstra
2019-11-20 10:13 ` Wanpeng Li
2019-11-05 23:51 ` Paolo Bonzini
2019-11-06 8:32 ` Peter Zijlstra
2019-11-06 9:41 ` Paolo Bonzini
2019-11-05 23:56 ` Paolo Bonzini
2019-12-06 4:01 ` Ankur Arora
2019-12-06 13:46 ` Vitaly Kuznetsov
2019-12-06 20:31 ` Ankur Arora
2019-12-09 9:15 ` Paolo Bonzini
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