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Date:   Wed, 4 Mar 2020 12:47:40 -0800
From:   Sean Christopherson <sean.j.christopherson@intel.com>
To:     Paolo Bonzini <pbonzini@redhat.com>
Cc:     Jan Kiszka <jan.kiszka@siemens.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org, Xiaoyao Li <xiaoyao.li@intel.com>
Subject: Re: [PATCH 3/6] KVM: x86: Add dedicated emulator helper for grabbing
 CPUID.maxphyaddr
Message-ID: <20200304204740.GG21662@linux.intel.com>
References: <20200302195736.24777-1-sean.j.christopherson@intel.com>
 <20200302195736.24777-4-sean.j.christopherson@intel.com>
 <de2ed4e9-409a-6cb1-e295-ea946be11e82@redhat.com>
 <617748ab-0edd-2ccc-e86b-b86b0adf9d3b@siemens.com>
 <4ddde497-9c71-d64c-df20-3b4439664336@redhat.com>
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On Tue, Mar 03, 2020 at 11:14:22AM +0100, Paolo Bonzini wrote:
> On 03/03/20 10:48, Jan Kiszka wrote:
> >>
> >> I don't think this is a particularly useful change.? Yes, it's not
> >> intuitive but is it more than a matter of documentation (and possibly
> >> moving the check_cr_write snippet into a separate function)?
> > 
> > Besides the non obvious return value of the current function, this
> > approach also avoids leaving cpuid traces for querying maxphyaddr, which
> > is also not very intuitive IMHO.
> 
> There are already other cases where we leave CPUID traces.  We can just
> stop tracing if check_limit (which should be renamed to from_guest) is
> true; there are other internal cases which call ctxt->ops->get_cpuid,
> such as vendor_intel, and those should also use check_limit==true and
> check the return value of ctxt->ops->get_cpuid.

No, the vendor checks that use get_cpuid() shouldn't do check_limit=true,
they're looking for an exact match on the vendor.

Not that it matters.  @check_limit only comes into play on a vendor check
if CPUID.0 doesn't exist, and @check_limit only effects the output if
CPUID.0 _does_ exist.  I.e. the output for CPUID.0 is unaffected by
@check_limit.