linux-kernel - Re: [PATCH] KVM: x86: Update Xen-specific CPUID leaves during mangling

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Message-ID: <Z5GXxOr3FHz_53Pj@google.com>
Date: Wed, 22 Jan 2025 17:13:40 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Vitaly Kuznetsov <vkuznets@...hat.com>
Cc: Fred Griffoul <fgriffo@...zon.co.uk>, kvm@...r.kernel.org, 
	Paolo Bonzini <pbonzini@...hat.com>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, 
	Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org, 
	"H. Peter Anvin" <hpa@...or.com>, David Woodhouse <dwmw2@...radead.org>, Paul Durrant <paul@....org>, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: x86: Update Xen-specific CPUID leaves during mangling

On Wed, Jan 22, 2025, Vitaly Kuznetsov wrote:
> > Signed-off-by: Fred Griffoul <fgriffo@...zon.co.uk>
> > ---
> >  arch/x86/kvm/cpuid.c | 1 +
> >  arch/x86/kvm/xen.c   | 5 +++++
> >  arch/x86/kvm/xen.h   | 5 +++++
> >  3 files changed, 11 insertions(+)
> >
> > diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> > index edef30359c19..432d8e9e1bab 100644
> > --- a/arch/x86/kvm/cpuid.c
> > +++ b/arch/x86/kvm/cpuid.c
> > @@ -212,6 +212,7 @@ static int kvm_cpuid_check_equal(struct kvm_vcpu *vcpu, struct kvm_cpuid_entry2
> >  	 */
> >  	kvm_update_cpuid_runtime(vcpu);
> >  	kvm_apply_cpuid_pv_features_quirk(vcpu);
> > +	kvm_xen_update_cpuid_runtime(vcpu);
> 
> This one is weird as we update it in runtime (kvm_guest_time_update())
> and values may change when we e.g. migrate the guest. First, I do not
> understand how the guest is supposed to notice the change as CPUID data
> is normally considered static.

I don't think it does.  Linux-as-a-guest reads the info once during boot (see
xen_tsc_safe_clocksource()), and if and only if the TSC is constant and non-stop,
i.e. iff the values won't change.  

>  Second, I do not see how the VMM is
> supposed to track it as if it tries to supply some different data for
> these Xen leaves, kvm_cpuid_check_equal() will still fail.
> 
> Would it make more sense to just ignore these Xen CPUID leaves with TSC
> information when we do the comparison?

Another alternative would be to modify the register output in kvm_cpuid().  Given
that Linux reads the info once during boot, and presumably other guests do the
same, runtime "patching" wouldn't incur meaningful overhead.  And there are no
feature bits that KVM cares about, i.e. no reason KVM's view needs to be correct.