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 12:12 PM, Sean Christopherson wrote:
> On Tue, Mar 03, 2020 at 12:02:39PM +0800, Xiaoyao Li wrote:
>> On 3/3/2020 11:45 AM, Sean Christopherson wrote:
>>> On Tue, Mar 03, 2020 at 10:27:47AM +0800, Xiaoyao Li wrote:
>>>> Sorry I cannot catch you. Why it's a violation of Intel's SDM?
>>>
>>> The case being discussed above would look like:
>>>
>>> KVM CPUID Entries:
>>>     Function   Index Output
>>>     0x00000000 0x00: eax=0x0000000b ebx=0x756e6547 ecx=0x6c65746e edx=0x49656e69
>>>     0x00000001 0x00: eax=0x000906ea ebx=0x03000800 ecx=0xfffa3223 edx=0x0f8bfbff
>>>     0x00000002 0x00: eax=0x00000001 ebx=0x00000000 ecx=0x0000004d edx=0x002c307d
>>>     0x00000003 0x00: eax=0x00000000 ebx=0x00000000 ecx=0x00000000 edx=0x00000000
>>>     0x00000004 0x00: eax=0x00000121 ebx=0x01c0003f ecx=0x0000003f edx=0x00000001
>>>     0x00000004 0x01: eax=0x00000122 ebx=0x01c0003f ecx=0x0000003f edx=0x00000001
>>>     0x00000004 0x02: eax=0x00000143 ebx=0x03c0003f ecx=0x00000fff edx=0x00000001
>>>     0x00000004 0x03: eax=0x00000163 ebx=0x03c0003f ecx=0x00003fff edx=0x00000006
>>>     0x00000005 0x00: eax=0x00000000 ebx=0x00000000 ecx=0x00000003 edx=0x00000000
>>>     0x00000006 0x00: eax=0x00000004 ebx=0x00000000 ecx=0x00000000 edx=0x00000000
>>>     0x00000007 0x00: eax=0x00000000 ebx=0x009c4fbb ecx=0x00000004 edx=0x84000000
>>>     0x00000008 0x00: eax=0x00000000 ebx=0x00000000 ecx=0x00000000 edx=0x00000000
>>>     0x00000009 0x00: eax=0x00000000 ebx=0x00000000 ecx=0x00000000 edx=0x00000000
>>>     0x0000000a 0x00: eax=0x07300402 ebx=0x00000000 ecx=0x00000000 edx=0x00000603
>>> --> MISSING CPUID.0xB.0
>>>     0x0000000b 0x01: eax=0x00000000 ebx=0x00000001 ecx=0x00000201 edx=0x00000003
>>>
>>> CPUID.0xB.0 does not exist, so output.ECX=0, which indicates an invalid
>>> level-type.
>>>
>>> The SDM states (for CPUID.0xB):
>>>
>>>     If an input value n in ECX returns the invalid level-type of 0 in ECX[15:8],
>>>     other input values with ECX > n also return 0 in ECX[15:8]
>>>
>>> That means returning a valid level-type in CPUID.0xB.1 as above violates
>>> the SDM's definition of how leaf 0xB works.  I'm arguing we can ignore the
>>> adjustments that would be done on output.E{C,D} for an out of range leaf
>>> because the model is bogus.
>>
>> Right.
>>
>> So we'd better do something in KVM_SET_CPUID* , to avoid userspace set bogus
>> cpuid.
>>
>>>> Supposing the max basic is 0x1f, and it queries cpuid(0x20, 0x5),
>>>> it should return cpuid(0x1f, 0x5).
>>>>
>>>> But based on this patch, it returns all zeros.
>>>
>>> Have you tested the patch, or is your comment based on the above discussion
>>> and/or code inspection?  Honest question, because I've thoroughly tested
>>> the above scenario and it works as you describe, but now I'm worried I
>>> completely botched my testing.
>>>
>>
>> No, I didn't test.
>>
>> Leaf 0xB and 0x1f are special cases when they are the maximum basic leaf,
>> because no matter what subleaf is, there is always a non-zero E[CX,DX].
>>
>> If cpuid.0 returns maximum basic leaf as 0xB/0x1F, when queried leaf is
>> greater, it should always return a non-zero value.
> 
> Yes, and that's userspace's responsibility to not screw up.  E.g. if
> userspace didn't create CPUID.0xB.0 (as above) then it's not KVM's fault
> for returning zeros when the guest executes CPUID.0xB.0.
> 

But this needs userspace to create all the subleaf of 0xB/0x1F. with a 
correct userspace, for example, it only creates one more subleaf of 
0xB/0x1F to indicate from this subleaf, no valid level-type anymore.

So when maximum basic leaf is 0x1f, cpuid(0x20, bigger than the first 
invalid subleaf created by userspace) returns all-zero.