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Date:   Mon, 3 Jun 2019 12:18:18 -0700
From:   Sean Christopherson <sean.j.christopherson@intel.com>
To:     Radim =?utf-8?B?S3LEjW3DocWZ?= <rkrcmar@redhat.com>
Cc:     Like Xu <like.xu@linux.intel.com>,
        Paolo Bonzini <pbonzini@redhat.com>, kvm@vger.kernel.org,
        Ingo Molnar <mingo@redhat.com>, linux-kernel@vger.kernel.org
Subject: Re: [RESEND PATCH v3] KVM: x86: Add Intel CPUID.1F cpuid emulation
 support
Message-ID: <20190603191818.GF13384@linux.intel.com>
References: <20190526133052.4069-1-like.xu@linux.intel.com>
 <20190603165616.GA11101@flask>
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On Mon, Jun 03, 2019 at 06:56:17PM +0200, Radim Krčmář wrote:
> > +			break;
> > +		}
> >  		entry->eax = min(entry->eax, (u32)(f_intel_pt ? 0x14 : 0xd));
> 
> Similarly in the existing code.  If we don't have f_intel_pt, then we
> should make sure that leaf 0x14 is not being filled, but we don't really
> have to limit the maximal index.
> 
> Adding a single clamping like
> 
> 		/* Limited to the highest leaf implemented in KVM. */
> 		entry->eax = min(entry->eax, 0x1f);
> 
> seems sufficient.
>
> (Passing the hardware value is ok in theory, but it is a cheap way to
>  avoid future leaves that cannot be simply zeroed for some weird reason.)

I don't have a strong opinion regarding the code itself, but whatever ends
up getting committed should have a big beefy changelog explaining why the
clamping exists, or at least extolling its virtues.  I had a hell of a
time understanding the intent of this one line of code because as your
response shows, there is no one right answer.