question about "struct kvm_segment"

question about "struct kvm_segment"

[Jidong Xiao]

Hi,

The structure kvm_segment is defined as follows:

struct kvm_segment {
        __u64 base;
        __u32 limit;
        __u16 selector;
        __u8  type;
        __u8  present, dpl, db, s, l, g, avl;
        __u8  unusable;
        __u8  padding;
};

Since the VMX spec defines base is 64 bits, limit is 32bits, selector
is 16bits, understandably, __u64, __u32, __u16 is used for them
respectively.

However, the VMX spec defines that the access rights is 32bits, which
includes a 4 bits type, 2 bits dpl, and 1 bit for the
present,dpl,db,s,l,g,avl. So why here they are all defined as __u8? It
means that, each of them is represented by 8bits? This looks
inconsistent with the VMX spec.

Hope someone can explain this, thanks!

-Jidong

Re: question about "struct kvm_segment"

[Paolo Bonzini]

Il 20/05/2014 04:25, Jidong Xiao ha scritto:
> Hi,
>
> The structure kvm_segment is defined as follows:
>
> struct kvm_segment {
>         __u64 base;
>         __u32 limit;
>         __u16 selector;
>         __u8  type;
>         __u8  present, dpl, db, s, l, g, avl;
>         __u8  unusable;
>         __u8  padding;
> };
>
> Since the VMX spec defines base is 64 bits, limit is 32bits, selector
> is 16bits, understandably, __u64, __u32, __u16 is used for them
> respectively.
>
> However, the VMX spec defines that the access rights is 32bits, which
> includes a 4 bits type, 2 bits dpl, and 1 bit for the
> present,dpl,db,s,l,g,avl. So why here they are all defined as __u8? It
> means that, each of them is represented by 8bits? This looks
> inconsistent with the VMX spec.

This is not a VMX definition, it's part of the ioctl interface between 
KVM and user-mode programs.

For the conversion code see vmx_get_segment and vmx_set_segment.  These 
should be never called in hot paths, except unfortunately when doing 
nested virtualization.

Paolo

Re: question about "struct kvm_segment"

[Jidong Xiao]

On Fri, May 30, 2014 at 4:49 PM, Paolo Bonzini <pbonzini@redhat.com> wrote:
> Il 20/05/2014 04:25, Jidong Xiao ha scritto:
>
>> Hi,
>>
>> The structure kvm_segment is defined as follows:
>>
>> struct kvm_segment {
>>         __u64 base;
>>         __u32 limit;
>>         __u16 selector;
>>         __u8  type;
>>         __u8  present, dpl, db, s, l, g, avl;
>>         __u8  unusable;
>>         __u8  padding;
>> };
>>
>> Since the VMX spec defines base is 64 bits, limit is 32bits, selector
>> is 16bits, understandably, __u64, __u32, __u16 is used for them
>> respectively.
>>
>> However, the VMX spec defines that the access rights is 32bits, which
>> includes a 4 bits type, 2 bits dpl, and 1 bit for the
>> present,dpl,db,s,l,g,avl. So why here they are all defined as __u8? It
>> means that, each of them is represented by 8bits? This looks
>> inconsistent with the VMX spec.
>
>
> This is not a VMX definition, it's part of the ioctl interface between KVM
> and user-mode programs.
>
> For the conversion code see vmx_get_segment and vmx_set_segment.  These
> should be never called in hot paths, except unfortunately when doing nested
> virtualization.
>
> Paolo

Thanks Paolo. I will try to read the source code and intel manual and
try to understand what you explained.

-Jidong