From: Sean Christopherson <sean.j.christopherson@intel.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: 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>,
Liran Alon <liran.alon@oracle.com>,
linux-kernel@vger.kernel.org, "H. Peter Anvin" <hpa@zytor.com>
Subject: Re: [PATCH RFC] KVM: x86: tell guests if the exposed SMT topology is trustworthy
Date: Tue, 5 Nov 2019 15:25:28 -0800 [thread overview]
Message-ID: <20191105232528.GF23297@linux.intel.com> (raw)
In-Reply-To: <20191105200218.GF3079@worktop.programming.kicks-ass.net>
On Tue, Nov 05, 2019 at 09:02:18PM +0100, Peter Zijlstra 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.
>
> The only way virt topology can make any sense what so ever is if the
> vcpus are pinned to physical CPUs.
>
> And I was under the impression we already had a bit for that (isn't it
> used to disable paravirt spinlocks and the like?). But I cannot seem to
> find it in a hurry.
Yep, KVM_HINTS_REALTIME does what you describe.
> So I would much rather you have a bit that indicates the 1:1 vcpu/cpu
> mapping and if that is set accept the topology information and otherwise
> completely ignore it.
next prev parent reply other threads:[~2019-11-05 23:25 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
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 [this message]
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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