linux-kernel - Re: [PATCH 48/61] KVM: x86: Do host CPUID at load time to mask KVM cpu caps

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Date:   Sat, 29 Feb 2020 10:38:30 -0800
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 48/61] KVM: x86: Do host CPUID at load time to mask KVM
 cpu caps

On Tue, Feb 25, 2020 at 01:08:43PM -0800, Sean Christopherson wrote:
> On Tue, Feb 25, 2020 at 04:18:12PM +0100, Paolo Bonzini wrote:
> > On 01/02/20 19:52, Sean Christopherson wrote:
> > > +#ifdef CONFIG_KVM_CPUID_AUDIT
> > > +	/* Entry needs to be fully populated when auditing is enabled. */
> > > +	entry.function = cpuid.function;
> > > +	entry.index = cpuid.index;
> > > +#endif
> > 
> > This shows that the audit case is prone to bitrot, which is good reason
> > to enable it by default.
> 
> I have no argument against that, especially since I missed this case during
> development and only caught it when running on a different system that I
> had happened to configure with CONFIG_KVM_CPUID_AUDIT=y. :-)

I ended up dropping the audit code altogether.  The uops overhead wasn't
bad, but the code bloat was pretty rough, ~16 bytes per instance.  The
final nail in the coffin was that the auditing would trigger false
positives if userspace configured CPUID leafs with a non-signficant index
to have a non-zero index, e.g. is_matching_cpuid_entry() ignores the index
if KVM_CPUID_FLAG_SIGNIFCANT_INDEX isn't set.