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

[Yang, Weijiang]

On 5/12/2022 9:18 PM, Paolo Bonzini wrote:
> 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.

Got it thanks!

But there's no such slot for MSR_ARCH_LBR_CTL in SMRAM, do you still suggest

using this mechanism to implement the LBREn clear/restore logic?

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