Date:   Thu, 25 Apr 2019 15:07:35 +0800
From:   Like Xu <like.xu@...ux.intel.com>
To:     Xiaoyao Li <xiaoyao.li@...el.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Len Brown <lenb@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: x86: Add Intel CPUID.1F cpuid emulation support

On 2019/4/25 14:30, Xiaoyao Li wrote:
> On Thu, 2019-04-25 at 14:02 +0800, Like Xu wrote:
>> On 2019/4/25 12:18, Xiaoyao Li wrote:
>>> On Thu, 2019-04-25 at 10:58 +0800, Like Xu wrote:
>>>> On 2019/4/24 22:32, Sean Christopherson wrote:
>>>>> Now that I understand how min() works...
>>>>>
>>>>> On Mon, Apr 22, 2019 at 02:40:34PM +0800, Like Xu wrote:
>>>>>> Expose Intel V2 Extended Topology Enumeration Leaf to guest only when
>>>>>> host system has multiple software-visible die within each package.
>>>>>>
>>>>>> Signed-off-by: Like Xu <like.xu@...ux.intel.com>
>>>>>> ---
>>>>>>     arch/x86/kvm/cpuid.c | 13 +++++++++++++
>>>>>>     1 file changed, 13 insertions(+)
>>>>>>
>>>>>> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
>>>>>> index fd39516..9fc14f2 100644
>>>>>> --- a/arch/x86/kvm/cpuid.c
>>>>>> +++ b/arch/x86/kvm/cpuid.c
>>>>>> @@ -65,6 +65,16 @@ u64 kvm_supported_xcr0(void)
>>>>>>     	return xcr0;
>>>>>>     }
>>>>>>     
>>>>>> +/* We need to check if the host cpu has multi-chip packaging
>>>>>> technology.
>>>>>> */
>>>>>> +static bool kvm_supported_intel_mcp(void)
>>>>>> +{
>>>>>> +	u32 eax, ignored;
>>>>>> +
>>>>>> +	cpuid_count(0x1f, 0, &eax, &ignored, &ignored, &ignored);
>>>>>
>>>>> This is broken because of how CPUID works for unsupported input leafs:
>>>>>
>>>>>      If a value entered for CPUID.EAX is higher than the maximum input
>>>>> value
>>>>>      for basic or extended function for that processor then the data for
>>>>> the
>>>>>      highest basic information leaf is returned.
>>>>>
>>>>> For example, my system with a max basic leaf of 0x16 returns 0x00000e74
>>>>> for CPUID.1F.EAX.
>>>>
>>>> You're right and the cpuid.1f.eax check is unreliable after I checked a
>>>> few machines.
>>>>
>>>>>
>>>>>> +
>>>>>> +	return boot_cpu_data.x86_vendor == X86_VENDOR_INTEL && (eax !=
>>>>>> 0);
>>>>>
>>>>> Checking 'eax != 0' is broken as it will be '0' when SMT is
>>>>> disabled.  ecx
>>>>> is the obvious choice since bits 15:8 are guaranteed to be non-zero when
>>>>> the leaf is valid.
>>>>
>>>> I agree with this and ecx[15:8] makes sense.
>>>>
>>>>>
>>>>> I think we can skip the vendor check.  AFAIK, CPUID.1F isn't used by
>>>>> AMD,
>>>>> and since AMD and Intel try to maintain a semblance of CPUID
>>>>> compatibility
>>>>> it seems more likely that AMD/Hygon would implement CPUID.1F as-is
>>>>> rather
>>>>> than repurpose it to mean something else entirely.
>>>>
>>>> If it's true, let's skip the vendor check.
>>>>
>>>> // I have to mention that AMD already has MCP CPUs.
>>>>
>>>>>
>>>>>> +}
>>>>>> +
>>>>>>     #define F(x) bit(X86_FEATURE_##x)
>>>>>>     
>>>>>>     int kvm_update_cpuid(struct kvm_vcpu *vcpu)
>>>>>> @@ -426,6 +436,7 @@ static inline int __do_cpuid_ent(struct
>>>>>> kvm_cpuid_entry2 *entry, u32 function,
>>>>>>     	switch (function) {
>>>>>>     	case 0:
>>>>>>     		entry->eax = min(entry->eax, (u32)(f_intel_pt ? 0x14 :
>>>>>> 0xd));
>>>>>> +		entry->eax = kvm_supported_intel_mcp() ? 0x1f : entry-
>>>>>>> eax;
>>>>>
>>>>> If we put everything together, I think the code can be reduced to:
>>>>>
>>>>> 		/* comment about multi-chip leaf... */
>>>>> 		if (entry->eax >= 0x1f && cpuid_ecx(0x1f))
>>>>> 			entry->eax = 0x1f;
>>>>> 		else
>>>>> 			entry->eax = min(entry->eax,
>>>>> 					 (u32)(f_intel_pt ? 0x14 :
>>>>> 0xd));
>>>>
>>>> Based on:
>>>>
>>>> 	ECX Bits 07 - 00: Level number. Same value in ECX input.
>>>> 	Bits 15 - 08: Level type.
>>>> 	Bits 31 - 16: Reserved.
>>>>
>>>> how about using an increasing order:
>>>>
>>>> 	entry->eax = min(entry->eax, (u32)(f_intel_pt ? 0x14 : 0xd));
>>>>
>>>> 	// ... more checks when eax is between 0x14 and 0x1f if any
>>>>
>>>> 	/* Check if the host cpu has multi-chip packaging technology.*/
>>>> 	if (((cpuid_ecx(0x1f) >> 8) & 0xff) != 0)
>>>> 		entry->eax = 0x1f;
>>>
>>> As Sean pointed out, you cannot rely on the output of cpuid.1f to indicate
>>> the
>>> existence of leaf 1f. If maximum basic leaf supported is smaller than 1f,
>>> the
>>> data returned by cpuid_ecx(0x1f) is the actual highest basic information
>>> leaf of
>>> the hardware.
>>
>> I don't think so.
>>
>>> So using "entry->eax >= 0x1f" from cpuid.0H is and only is the right way to
>>> check the existence of leaf 1f.
>>>
>>> We can simply use (cpuid_ecx(0x1f) & 0x0000ff00) to avoid the unnecessory
>>> shifting operation.
>>
>> I borrowed this "unnecessory" shifting operation from host
>> check_extended_topology_leaf() and we may do better on this.
>>
>>> Besides, the problem of simply using cpuid_exc(0x1f) in Sean's codes is that
>>> we
>>> cannot assmue the reserved bits 31:16 of ECX is always 0 for the future
>>> generation.
>>
>> It's true cause the statement in public spec is not "Reserved = 0" but
>> "Bits 31 - 16: Reserved".
>>
>>>
>>> In my opinion, Sean's codes is OK and much simple and clear.
>>
>> If the host cpuid.0.eax is greater than 0x1f but actually it doesn't
>> have multi-chip packaging technology and we may want to expose
>> entry->eax to some value smaller than 0x1f but greater than 0x14, much
>> effort needs to apply on Sean's code.
>>
>> My improvement is good to overwrite cpuid.0.eax in future usage
>> from the perspective of kvm feature setting not just from value check.
> 
> Alright, there is something wrong in your code that you haven't realised.
> 
> When you do
> 	entry->eax = min(entry->eax, (u32)(f_intel_pt ? 0x14 : 0xd));
> 
> it changes the entry->eax if entry->eax > 0x14. So you cannot directly use
> cpuid_ecx(0x1f). At least, you need to cache the value of entry->eax, like:
> 	
> 	u32 max_leaf = entry->eax;
> 	entry->eax = min(entry->eax, (u32)(f_intel_pt ? 0x14 : 0xd));
> 	
> 	//...leaf between 0x14 and 0x1f
> 	
> 	if (max_leaf >= 0x1f && (cpuid_ecx(0x1f) & 0x0000ff00))
>   		entry->eax = 0x1f;

The cache value make no sense on this.

> 
> However, handling in increasing order in totally wrong. Since it's to report the
> max the leaf supported, we should handle in descending order, which is what Sean
> does.

There is no need to check "entry->eax >= 0x1f" before "setting 
entry->eax = 0x1f" if and only if cpuid_ecx(0x1f) meets requirements.

An increasing manner helps to overwrite this value on demand in a flat 
code flow (easy to understand and maintain) not an if-else_if-else flow.

> 
>>> All need to do is using (cpuid_ecx(0x1f) & 0x0000ff00) to verify the leaf.1f
>>> is
>>> valid.
>>>
>>> Thanks,
>>> -Xiaoyao
>>>> 	// ... more checks when eax greater than 0x1f if any
>>>>
>>>> are we OK with it?
>>>>
>>>>>>     		break;
>>>>>>     	case 1:
>>>>>>     		entry->edx &= kvm_cpuid_1_edx_x86_features;
>>>>>> @@ -544,6 +555,8 @@ static inline int __do_cpuid_ent(struct
>>>>>> kvm_cpuid_entry2 *entry, u32 function,
>>>>>>     		entry->edx = edx.full;
>>>>>>     		break;
>>>>>>     	}
>>>>>> +	/* function 0x1f has additional index. */
>>>>>> +	case 0x1f:
>>>>>>     	/* function 0xb has additional index. */
>>>>>>     	case 0xb: {
>>>>>>     		int i, level_type;
>>>>>> -- 
>>>>>> 1.8.3.1
>>>>>>
>>>>
>>>>
>>>
>>>
>>
>>
> 
> 

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