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
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
On Fri, May 30, 2014 at 4:49 PM, Paolo Bonzini <pbonzini at 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. > > PaoloThanks Paolo. I will try to read the source code and intel manual and try to understand what you explained. -Jidong