linux-kernel - Re: [PATCH v2 2/5] KVM: SVM: allow to force AVIC to be enabled

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Date:   Wed, 05 Jan 2022 12:54:19 +0200
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     kvm@...r.kernel.org, Jim Mattson <jmattson@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Joerg Roedel <joro@...tes.org>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Borislav Petkov <bp@...en8.de>, linux-kernel@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH v2 2/5] KVM: SVM: allow to force AVIC to be enabled

On Tue, 2022-01-04 at 22:25 +0000, Sean Christopherson wrote:
> On Mon, Dec 13, 2021, Maxim Levitsky wrote:
> > Apparently on some systems AVIC is disabled in CPUID but still usable.
> > 
> > Allow the user to override the CPUID if the user is willing to
> > take the risk.
> > 
> > Signed-off-by: Maxim Levitsky <mlevitsk@...hat.com>
> > ---
> >  arch/x86/kvm/svm/svm.c | 11 +++++++++--
> >  1 file changed, 9 insertions(+), 2 deletions(-)
> > 
> > diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> > index c9668a3b51011..468cc385c35f0 100644
> > --- a/arch/x86/kvm/svm/svm.c
> > +++ b/arch/x86/kvm/svm/svm.c
> > @@ -206,6 +206,9 @@ module_param(tsc_scaling, int, 0444);
> >  static bool avic;
> >  module_param(avic, bool, 0444);
> >  
> > +static bool force_avic;
> > +module_param_unsafe(force_avic, bool, 0444);
> > +
> >  bool __read_mostly dump_invalid_vmcb;
> >  module_param(dump_invalid_vmcb, bool, 0644);
> >  
> > @@ -4656,10 +4659,14 @@ static __init int svm_hardware_setup(void)
> >  			nrips = false;
> >  	}
> >  
> > -	enable_apicv = avic = avic && npt_enabled && boot_cpu_has(X86_FEATURE_AVIC);
> > +	enable_apicv = avic = avic && npt_enabled && (boot_cpu_has(X86_FEATURE_AVIC) || force_avic);
> >  
> >  	if (enable_apicv) {
> > -		pr_info("AVIC enabled\n");
> > +		if (!boot_cpu_has(X86_FEATURE_AVIC)) {
> > +			pr_warn("AVIC is not supported in CPUID but force enabled");
> > +			pr_warn("Your system might crash and burn");
> > +		} else
> 
> Needs curly braces, though arguably the "AVIC enabled" part should be printed
> regardless of boot_cpu_has(X86_FEATURE_AVIC).
> 
> > +			pr_info("AVIC enabled\n");
> 
> This is all more than a bit terrifying, though I can see the usefuless for a
> developer.  At the very least, this should taint the kernel.  This should also
> probably be buried behind a Kconfig that is itself buried behind EXPERT.
> 
I used 'module_param_unsafe' which does taint the kernel.

Best regards,
	Maxim Levitsky