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

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Message-ID: <7a8eb4a3-c58a-fd32-4419-4e799870c757@intel.com>
Date:   Thu, 12 May 2022 22:38:42 +0800
From:   "Yang, Weijiang" <weijiang.yang@...el.com>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        "jmattson@...gle.com" <jmattson@...gle.com>,
        "seanjc@...gle.com" <seanjc@...gle.com>,
        "kan.liang@...ux.intel.com" <kan.liang@...ux.intel.com>,
        "like.xu.linux@...il.com" <like.xu.linux@...il.com>,
        "vkuznets@...hat.com" <vkuznets@...hat.com>,
        "Wang, Wei W" <wei.w.wang@...el.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v11 14/16] KVM: x86/vmx: Flip Arch LBREn bit on guest
 state change


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!