Re: [PATCH v11 14/16] KVM: x86/vmx: Flip Arch LBREn bit on guest state change

[Paolo Bonzini]

On 5/11/22 09:43, Yang, Weijiang wrote:
> > Instead of using flip_arch_lbr_ctl, SMM should save the value of the MSR
> > in kvm_x86_ops->enter_smm, and restore it in kvm_x86_ops->leave_smm
> > (feel free to do it only if guest_cpuid_has(vcpu, X86_FEATURE_LM), i.e.
> > the 32-bit case can be ignored).
>
> In the case of migration in SMM, I assume kvm_x86_ops->enter_smm() 
> called in source side
>
> and kvm_x86_ops->leave_smm() is called at destination, then should the 
> saved LBREn be transferred
>
> to destination too? The destination can rely on the bit to defer setting 
> LBREn bit in

Hi, it's transferred automatically if the MSR is saved in the SMM save 
state area.  Both enter_smm and leave_smm can access the save state area.

The enter_smm callback is called after saving "normal" state, and it has 
to save the state + clear the bit; likewise, the leave_smm callback is 
called before saving "normal" state and will restore the old value of 
the MSR.

Thanks,

Paolo

> VMCS until kvm_x86_ops->leave_smm() is called. is it good? thanks!