linux-kernel - Re: [PATCH v3 3/6] KVM: x86: Hyper-V invariant TSC control

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [day] [month] [year] [list]

Message-ID: <Yyset3mEVytKdWd8@google.com>
Date:   Wed, 21 Sep 2022 14:32:44 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Michael Kelley <mikelley@...rosoft.com>,
        Maxim Levitsky <mlevitsk@...hat.com>,
        linux-hyperv@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 3/6] KVM: x86: Hyper-V invariant TSC control

On Wed, Sep 21, 2022, Vitaly Kuznetsov wrote:
> Sean Christopherson <seanjc@...gle.com> writes:
> > It's still not obvious to me why KVM shouldn't do:
> >
> > 	if (!hv_vcpu)
> > 		return false;
> >
> > 	return !(hv_vcpu->cpuid_cache.features_eax & HV_ACCESS_TSC_INVARIANT) ||
> > 	       !(to_kvm_hv(vcpu->kvm)->hv_invtsc_control & HV_INVARIANT_TSC_EXPOSED);
> >
> > I.e. why is invariant TSC _not_ suppressed on Hyper-V by default?
> 
> In case we switch to suppressing invtsc (CPUID.80000007H:EDX[8]) by
> default, i.e. when HV_ACCESS_TSC_INVARIANT was not set in guest visible
> CPUIDs, this is going to be a behavioral change for the already existing
> configurations and we certainly don't want that. It was expirementally
> proven that at least some Windows versions are perfectly happy when they
> see invtsc without this PV feature so I don't see a need to break the
> status quo.
> 
> this is VMM's job, not KVM's. 

Gotcha.  Can you add a comment to capture this?  In particular, the part about it
being KVM's responsibility iff the Hyper-V control is exposed to the guest.

Hmm, and I think it makes to repackage the code so that the "is KVM responsible"
check is separated from the "is the control enabled".  E.g.

	/*
	 * If Hyper-V's invariant TSC control is exposed to the guest, KVM is
	 * responsible for suppressing the invariant TSC CPUID flag if the
	 * Hyper-V control is not enabled.
	 */
	if (!hv_vcpu ||
	    !(hv_vcpu->cpuid_cache.features_eax & HV_ACCESS_TSC_INVARIANT))
		return false;

	return !(to_kvm_hv(vcpu->kvm)->hv_invtsc_control & HV_INVARIANT_TSC_EXPOSED);