Re: [RFC PATCH 4/4] KVM: vmx: add support for emulating UMIP

[Paolo Bonzini <pbonzini@redhat.com>]

On 31/07/2016 04:32, Wanpeng Li wrote:
> 2016-07-14 16:09 GMT+08:00 Paolo Bonzini <pbonzini@redhat.com>:
> [...]
>>
>> This is not necessary because this is how KVM computes
>> CPUID[EAX=7,EBX=0].ECX:
>>
>>         unsigned f_umip = kvm_x86_ops->umip_emulated() ? F(UMIP) : 0;
>>         ...
>>         const u32 kvm_cpuid_7_0_ecx_x86_features = F(PKU) | F(UMIP);
>>         ...
>>         // Mask userspace-provided value against supported features
>>         entry->ecx &= kvm_cpuid_7_0_ecx_x86_features;
>>         // Mask userspace-provided value against host features
>>         cpuid_mask(&entry->ecx, CPUID_7_ECX);
>>         // Finally add emulated features
>>         entry->ecx |= f_umip;
> 
> I think you mean:
> 
> - entry->ecx  ->  userspace-provided value
> - kvm_cpuid_7_0_ecx_x86_features  ->  supported features
> - CPUID_7_ECX  ->  host features
> 
> However, entry->ecx is returned by cpuid instruction
> (do_cpuid_1_ent()), so why it is a userspace-provided value?

You're right, it's this:

         // Mask host processor value against supported features
         entry->ecx &= kvm_cpuid_7_0_ecx_x86_features;
         // Mask host processor value further, e.g. to drop
	 // features that the host kernel has blacklisted.
         cpuid_mask(&entry->ecx, CPUID_7_ECX);
         // Finally add emulated features
         entry->ecx |= f_umip;

The idea is the same. :)

On the other hand, it is true that in many cases of the "switch
(function)" the call to do_cpuid_1_ent is unnecessary, and instead of
cpuid_mask you could just access boot_cpu_data.x86_capability[wordnum].

Paolo