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Date:   Wed, 6 Nov 2019 09:32:35 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Sean Christopherson <sean.j.christopherson@intel.com>
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
Message-ID: <20191106083235.GP4131@hirez.programming.kicks-ass.net>
References: <20191105161737.21395-1-vkuznets@redhat.com>
 <20191105200218.GF3079@worktop.programming.kicks-ass.net>
 <20191105232528.GF23297@linux.intel.com>
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On Tue, Nov 05, 2019 at 03:25:28PM -0800, Sean Christopherson wrote:
> 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.

*sigh*, that's a pretty shit name for it :/