Re: [PATCH 1/6] KVM: x86: Fix tracing of CPUID.function when function is out-of-range

[Xiaoyao Li <xiaoyao.li@intel.com>]

On 3/3/2020 12:08 PM, Sean Christopherson wrote:
> On Tue, Mar 03, 2020 at 10:50:03AM +0800, Xiaoyao Li wrote:
>> On 3/3/2020 3:57 AM, Sean Christopherson wrote:
>>> Rework kvm_cpuid() to query entry->function when adjusting the output
>>> values so that the original function (in the aptly named "function") is
>>> preserved for tracing.  This fixes a bug where trace_kvm_cpuid() will
>>> trace the max function for a range instead of the requested function if
>>> the requested function is out-of-range and an entry for the max function
>>> exists.
>>>
>>> Fixes: 43561123ab37 ("kvm: x86: Improve emulation of CPUID leaves 0BH and 1FH")
>>> Reported-by: Jan Kiszka <jan.kiszka@siemens.com>
>>> Cc: Jim Mattson <jmattson@google.com>
>>> Cc: Xiaoyao Li <xiaoyao.li@intel.com>
>>> Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
>>> ---
>>>   arch/x86/kvm/cpuid.c | 15 +++++++--------
>>>   1 file changed, 7 insertions(+), 8 deletions(-)
>>>
>>> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
>>> index b1c469446b07..6be012937eba 100644
>>> --- a/arch/x86/kvm/cpuid.c
>>> +++ b/arch/x86/kvm/cpuid.c
>>> @@ -997,12 +997,12 @@ static bool cpuid_function_in_range(struct kvm_vcpu *vcpu, u32 function)
>>>   	return max && function <= max->eax;
>>>   }
>>> +/* Returns true if the requested leaf/function exists in guest CPUID. */
>>>   bool kvm_cpuid(struct kvm_vcpu *vcpu, u32 *eax, u32 *ebx,
>>>   	       u32 *ecx, u32 *edx, bool check_limit)
>>>   {
>>> -	u32 function = *eax, index = *ecx;
>>> +	const u32 function = *eax, index = *ecx;
>>>   	struct kvm_cpuid_entry2 *entry;
>>> -	struct kvm_cpuid_entry2 *max;
>>>   	bool found;
>>>   	entry = kvm_find_cpuid_entry(vcpu, function, index);
>>> @@ -1015,18 +1015,17 @@ bool kvm_cpuid(struct kvm_vcpu *vcpu, u32 *eax, u32 *ebx,
>>>   	 */
>>>   	if (!entry && check_limit && !guest_cpuid_is_amd(vcpu) &&
>>>   	    !cpuid_function_in_range(vcpu, function)) {
>>> -		max = kvm_find_cpuid_entry(vcpu, 0, 0);
>>> -		if (max) {
>>> -			function = max->eax;
>>> -			entry = kvm_find_cpuid_entry(vcpu, function, index);
>>> -		}
>>> +		entry = kvm_find_cpuid_entry(vcpu, 0, 0);
>>> +		if (entry)
>>> +			entry = kvm_find_cpuid_entry(vcpu, entry->eax, index);
>>
>> There is a problem.
>>
>> when queried leaf is out of range on Intel CPU, it returns the maximum basic
>> leaf, and any dependence on input ECX (i.e., subleaf) value in the basic
>> leaf is honored. As disclaimed in SDM of CPUID instruction.
> 
> That's what the code above does.
> 
>> The ECX should be honored if and only the leaf has a significant index.
>> If the leaf doesn't has a significant index, it just ignores the EDX input
> 
> s/EDX/ECX
> 
>> in bare metal.
>>
>> So it should be something like:
>>
>> if (!entry && check_limit && !guest_cpuid_is_amd(vcpu) &&
>> 	!cpuid_function_in_range(vcpu, function)) {
>> 	entry = kvm_find_cpuid_entry(vcpu, 0, 0);
>> 	if (entry) {
>> 		entry = kvm_find_cpuid_entry(vcpu, entry->eax, 0);
>> 		if (entry &&
>> 		    entry->flags & KVM_CPUID_FLAG_SIGNIFCANT_INDEX ) {
> 
> This is unnecessary IMO.  The only scenario where SIGNFICANT_INDEX is 0
> and cpuid_entry(entry->eax, 0) != cpuid_entry(entry->eax, index) is if
> userspace created a cpuid entry for index>0 with SIGNFICANT_INDEX.  

I just forgot that is_matching_cpuid_entry() has taken SIGNIFICANT_INDEX 
into account.

Please ignore my stupid noise.

> a busted model, e.g. it'd be the SDM equivalent of an Intel CPU having
> different output for CPUID.0x16.0 and CPUID.16.5 despite the SDM stating
> that the CPUID.0x16 ignores the index.
> 
> E.g. on my system with a max basic leaf of 0x16
> 
> $ cpuid -1 -r
> CPU:
>     0x00000000 0x00: eax=0x00000016 ebx=0x756e6547 ecx=0x6c65746e edx=0x49656e69
> ...
>     0x00000016 0x00: eax=0x00000e74 ebx=0x0000125c ecx=0x00000064 edx=0x00000000
> 
> $ cpuid -1 -r -l 0x16
> CPU:
>     0x00000016 0x00: eax=0x00000e74 ebx=0x0000125c ecx=0x00000064 edx=0x00000000
> ~ $ cpuid -1 -r -l 0x16 -s 4
> CPU:
>     0x00000016 0x04: eax=0x00000e74 ebx=0x0000125c ecx=0x00000064 edx=0x00000000
> ~ $ cpuid -1 -r -l 0x16 -s 466
> CPU:
>     0x00000016 0x1d2: eax=0x00000e74 ebx=0x0000125c ecx=0x00000064 edx=0x00000000
> 
> 
> If it returned anything else for CPUID.0x16.0x4 then it'd be a CPU bug.
> Same thing here, it's a userspace bug if it creates a CPUID entry that
> shouldn't exist.  E.g. ignoring Intel's silly "max basic leaf" behavior
> for the moment, if userspace created a entry for CPUID.0x0.N it would
> break the Linux kernel's cpu_detect(), as it doesn't initialize ECX when
> doing CPUID.0x0.
> 
>> 			entry = kvm_find_cpuid_entry(vcpu, entry->eax,
>> 						     index);
>> 		}
>> 	}
>> }
>>
>>>   	}
>>>   	if (entry) {
>>>   		*eax = entry->eax;
>>>   		*ebx = entry->ebx;
>>>   		*ecx = entry->ecx;
>>>   		*edx = entry->edx;
>>> -		if (function == 7 && index == 0) {
>>> +
>>> +		if (entry->function == 7 && index == 0) {
>>>   			u64 data;
>>>   		        if (!__kvm_get_msr(vcpu, MSR_IA32_TSX_CTRL, &data, true) &&
>>>   			    (data & TSX_CTRL_CPUID_CLEAR))
>>>
>>