linux-kernel - Re: [PATCH 5/6] KVM: x86: Rename "found" variable in kvm_cpuid() to "exact_entry

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Date:   Mon, 2 Mar 2020 21:48:26 +0100
From:   Jan Kiszka <jan.kiszka@...mens.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        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, Xiaoyao Li <xiaoyao.li@...el.com>
Subject: Re: [PATCH 5/6] KVM: x86: Rename "found" variable in kvm_cpuid() to
 "exact_entry_exists"

On 02.03.20 21:35, Sean Christopherson wrote:
> On Mon, Mar 02, 2020 at 09:20:52PM +0100, Jan Kiszka wrote:
>> On 02.03.20 20:57, Sean Christopherson wrote:
>>> Rename "found" in kvm_cpuid() to "exact_entry_exists" to better convey
>>> that the intent of the tracepoint's "found/not found" output is to trace
>>> whether the output values are for the actual requested leaf or for some
>>> other (likely unrelated) leaf that was found while processing entries to
>>> emulate funky CPU behavior, e.g. the max basic leaf on Intel CPUs when
>>> the requested CPUID leaf is out of range.
>>>
>>> Suggested-by: Jan Kiszka <jan.kiszka@...mens.com>
>>> Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>
>>> ---
>>>   arch/x86/kvm/cpuid.c | 6 +++---
>>>   1 file changed, 3 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
>>> index 869526930cf7..b0a4f3c17932 100644
>>> --- a/arch/x86/kvm/cpuid.c
>>> +++ b/arch/x86/kvm/cpuid.c
>>> @@ -1002,10 +1002,10 @@ void kvm_cpuid(struct kvm_vcpu *vcpu, u32 *eax, u32 *ebx,
>>>   {
>>>   	const u32 function = *eax, index = *ecx;
>>>   	struct kvm_cpuid_entry2 *entry;
>>> -	bool found;
>>> +	bool exact_entry_exists;
>>>   	entry = kvm_find_cpuid_entry(vcpu, function, index);
>>> -	found = entry;
>>> +	exact_entry_exists = !!entry;
>>>   	/*
>>>   	 * Intel CPUID semantics treats any query for an out-of-range
>>>   	 * leaf as if the highest basic leaf (i.e. CPUID.0H:EAX) were
>>> @@ -1047,7 +1047,7 @@ void kvm_cpuid(struct kvm_vcpu *vcpu, u32 *eax, u32 *ebx,
>>>   			}
>>>   		}
>>>   	}
>>> -	trace_kvm_cpuid(function, *eax, *ebx, *ecx, *edx, found);
>>> +	trace_kvm_cpuid(function, *eax, *ebx, *ecx, *edx, exact_entry_exists);
>>
>> Actually, I think we also what to change output in the tracepoint.
> 
> Oh, I definitely want to change it, but AIUI it's ABI and shouldn't be
> changed.  Paolo?
> 

This can be discovered via the format string in sysfs and handled by the 
consumer. But I'm not an expert in the tracing ABI.

Jan

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