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

Started byWanpeng Li <kernellwp@gmail.com>
First post2016-07-31 04:40 +0200
Last post2016-08-02 09:00 +0200
Articles 3 — 2 participants

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  Re: [RFC PATCH 4/4] KVM: vmx: add support for emulating UMIP Wanpeng Li <kernellwp@gmail.com> - 2016-07-31 04:40 +0200
    Re: [RFC PATCH 4/4] KVM: vmx: add support for emulating UMIP Paolo Bonzini <pbonzini@redhat.com> - 2016-08-01 17:10 +0200
      Re: [RFC PATCH 4/4] KVM: vmx: add support for emulating UMIP Wanpeng Li <kernellwp@gmail.com> - 2016-08-02 09:00 +0200

#1452764 — Re: [RFC PATCH 4/4] KVM: vmx: add support for emulating UMIP

FromWanpeng Li <kernellwp@gmail.com>
Date2016-07-31 04:40 +0200
SubjectRe: [RFC PATCH 4/4] KVM: vmx: add support for emulating UMIP
Message-ID<s0OPv-2vt-5@gated-at.bofh.it>
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?

Regards,
Wanpeng Li

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#1453267

FromPaolo Bonzini <pbonzini@redhat.com>
Date2016-08-01 17:10 +0200
Message-ID<s1n0S-7W5-17@gated-at.bofh.it>
In reply to#1452764

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

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#1453622

FromWanpeng Li <kernellwp@gmail.com>
Date2016-08-02 09:00 +0200
Message-ID<s1BQe-TD-9@gated-at.bofh.it>
In reply to#1453267
2016-08-01 23:01 GMT+08:00 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;
>

Cool, more clear this time. :)

> 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].

Agreed.

Regards,
Wanpeng Li

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