linux-kernel - Re: [PATCH v2] KVM: x86: Add Intel CPUID.1F cpuid emulation support

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Message-ID: <9c374edc-0002-de46-60f0-5f36b26c898c@linux.intel.com>
Date:   Fri, 26 Apr 2019 23:32:45 +0800
From:   Like Xu <like.xu@...ux.intel.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>,
        Xiaoyao Li <xiaoyao.li@...ux.intel.com>
Cc:     kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] KVM: x86: Add Intel CPUID.1F cpuid emulation support

On 2019/4/26 23:04, Sean Christopherson wrote:
> On Fri, Apr 26, 2019 at 10:54:19PM +0800, Xiaoyao Li wrote:
>> On Fri, 2019-04-26 at 07:13 -0700, Sean Christopherson wrote:
>>> On Fri, Apr 26, 2019 at 11:17:52AM +0800, Like Xu wrote:
>>>> Some new systems have multiple software-visible die within each package.
>>>> Add support to expose Intel V2 Extended Topology Enumeration Leaf CPUID.1F.
>>>>
>>>> Co-developed-by: Xiaoyao Li <xiaoyao.li@...ux.intel.com>
>>>> Signed-off-by: Xiaoyao Li <xiaoyao.li@...ux.intel.com>
>>>> Signed-off-by: Like Xu <like.xu@...ux.intel.com>
>>>> ---
>>>>
>>>> ==changelog==
>>>> v2:
>>>> - Apply cpuid.1f check rule on Intel SDM page 3-222 Vol.2A
>>>> - Add comment to handle 0x1f anf 0xb in common code
>>>> - Reduce check time in a descending-break style
>>>>
>>>> v1: https://lkml.org/lkml/2019/4/22/28
>>>>
>>>>   arch/x86/kvm/cpuid.c | 12 +++++++++++-
>>>>   1 file changed, 11 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
>>>> index fd39516..f9b529e 100644
>>>> --- a/arch/x86/kvm/cpuid.c
>>>> +++ b/arch/x86/kvm/cpuid.c
>>>> @@ -425,6 +425,11 @@ static inline int __do_cpuid_ent(struct
>>>> kvm_cpuid_entry2 *entry, u32 function,
>>>>   
>>>>   	switch (function) {
>>>>   	case 0:
>>>> +		/* Check if the cpuid leaf 0x1f is actually implemented */
>>>> +		if (entry->eax >= 0x1f && (cpuid_ebx(0x1f) & 0x0000ffff)) {
>>>
>>> Restricting the check to bits 15:0 is unnecessary, the SDM explicitly
>>> states that EBX will be zero for invalid sub-leaves:
>>>
>>>    For sub-leaves that return an invalid level-type of 0 in ECX[15:8];
>>>    EAX and EBX will return 0.
>>>
>>> This code is merely checking for the existence of CPUID.1F, nothing will
>>> break if future CPUs provide additional information, i.e. checking for a
>>> valid sub-leaf is sufficient.
>>>
>>> That being said, if you insist on restricting the check to non-reserved
>>> bits then I think the earlier suggestion of "cpuid_ecx(0x1f) & 0x0000ff00"
>>> makes more sense since the SDM clearly intends ECX to be used to detect
>>> valid vs. invalid levels.
>>
>> Here we use CPUID.1F_0:EBX[15:0] to check the existence, not the output of
>> ECX[15:8], which is following the Intel SDM.
>>
>> Specifically, in page 3-222 Vol.2A of latest SDM publish on January 2019, there
>> is such description of Input EAX = 1FH:
>>
>>     When CPUID executes with EAX set to 1FH, the processor returns information
>>     about extended topology enumeration data. Software must detect the presence
>>     of CPUID leaf 1FH by verifying (a) the highest leaf index supported by CPUID
>>     is >= 1FH, and (b) CPUID.1FH:EBX[15:0] reports a non-zero value.
> 
> Ah, perfect.  Please put exactly that in the changelog.

Thanks Sean and xiaoyao.
Both of you inspired me a lot and a v3 patch would be released.

> 
>>
>> And also, I look up the existing codes about initialising topology using leaf
>> 0xb in kernel. In function detect_extended_topology_early(), it use following
>>
>>     if (ebx == 0 || (LEAFB_SUBTYPE(ecx) != SMT_TYPE))
>>
>> to verify whether leaf 0xb is invalid. I think any of them is strong enough to
>> verify the leaf 0xb is invalid, so I don't know why it uses both of them.
>>
>>>> +			entry->eax = 0x1f;
>>>> +			break;
>>>
>>> I find if/else easier to follow than the separate break, but either option
>>> works for me.
>>> 		
>>>> +		}
>>>>   		entry->eax = min(entry->eax, (u32)(f_intel_pt ? 0x14 : 0xd));
>>>>   		break;
>>>>   	case 1:
>>>> @@ -544,7 +549,12 @@ static inline int __do_cpuid_ent(struct
>>>> kvm_cpuid_entry2 *entry, u32 function,
>>>>   		entry->edx = edx.full;
>>>>   		break;
>>>>   	}
>>>> -	/* function 0xb has additional index. */
>>>> +	/*
>>>> +	 * Intel documentation states that 0x1f and 0xb have
>>>> +	 * identical formats and thus can be handled by common code.
>>>> +	 * (Intel SDM Vol. 2A - Instruction Set Reference - CPUID)
>>>> +	 */
>>>
>>> It's probably safe to assume anyone reading reading this code is already
>>> all too aware of Intel's propensity for dumping CPUID enumeration into the
>>> CPUID entry in the ISR.  Maybe shorten this to something like:
>>>
>>> 	/*
>>> 	 * Per Intel's SDM, 0x1f is a superset of 0xb, thus they can be handled
>>> 	 * by common code.
>>> 	 */
>>>
>>>> +	case 0x1f:
>>>>   	case 0xb: {
>>>>   		int i, level_type;
>>>>   
>>>> -- 
>>>> 1.8.3.1
>>>>
>>
>