linux-kernel - Re: [PATCH v3 7/9] KVM: nSVM: implement nested_svm_load

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Message-ID: <878sfrv9g3.fsf@vitty.brq.redhat.com>
Date:   Fri, 10 Jul 2020 14:30:36 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org
Cc:     Sean Christopherson <sean.j.christopherson@...el.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Junaid Shahid <junaids@...gle.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 7/9] KVM: nSVM: implement nested_svm_load_cr3() and use it for host->guest switch

Paolo Bonzini <pbonzini@...hat.com> writes:

> On 10/07/20 13:40, Vitaly Kuznetsov wrote:
>> Hm, it seems I missed svm_set_nested_state() path
>> completely. Surprisingly, state_test didn't fail)
>> 
>> I'm struggling a bit to understand why we don't have kvm_set_cr3() on
>> svm_set_nested_state() path: enter_svm_guest_mode() does it through
>> nested_prepare_vmcb_save() but it is skipped in svm_set_nested_state().
>> Don't we need it at least for !npt_enabled case?
>
> In svm_set_nested_state you'll have CR3 already set to the right value.
>  On the source, KVM_GET_SREGS returns the vmcb12's CR3 and it is already
> restored with KVM_SET_SREGS on the destination before set_nested_state.
>
> So, only the nested_cr3 has to be set.
>

Ah, thanks, so it seems there is no need to merge nested_svm_load_cr3()
(former kvm_set_cr3()) with nested_svm_init_mmu_context() in a new
nested_svm_init_mmu() as we'll only need the latter on
svm_set_nested_state(). We can just move nested_svm_load_cr3() out of
nested_prepare_vmcb_save() to enter_svm_guest_mode(). 

-- 
Vitaly