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[35.185.214.157]) by smtp.gmail.com with ESMTPSA id s123sm8482257pfb.78.2021.06.18.13.53.52 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 18 Jun 2021 13:53:52 -0700 (PDT) Date: Fri, 18 Jun 2021 20:53:48 +0000 From: Sean Christopherson To: Maxim Levitsky Cc: kvm@vger.kernel.org, Paolo Bonzini , open list , "open list:DOCUMENTATION" , Wanpeng Li , Ingo Molnar , Vitaly Kuznetsov , Thomas Gleixner , Borislav Petkov , Joerg Roedel , "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" , Jim Mattson , Jonathan Corbet , "H. Peter Anvin" Subject: Re: [PATCH v3 8/8] KVM: x86: avoid loading PDPTRs after migration when possible Message-ID: References: <20210607090203.133058-1-mlevitsk@redhat.com> <20210607090203.133058-9-mlevitsk@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210607090203.133058-9-mlevitsk@redhat.com> Precedence: bulk List-ID: X-Mailing-List: linux-doc@vger.kernel.org > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c > index 11260e83518f..eadfc9caf500 100644 > --- a/arch/x86/kvm/x86.c > +++ b/arch/x86/kvm/x86.c > @@ -815,6 +815,8 @@ int load_pdptrs(struct kvm_vcpu *vcpu, struct kvm_mmu *mmu, unsigned long cr3) > > memcpy(mmu->pdptrs, pdpte, sizeof(mmu->pdptrs)); > kvm_register_mark_dirty(vcpu, VCPU_EXREG_PDPTR); > + vcpu->arch.pdptrs_restored_oob = false; > + > out: > > return ret; > @@ -10113,6 +10115,7 @@ static int __set_sregs2(struct kvm_vcpu *vcpu, struct kvm_sregs2 *sregs2) > > kvm_register_mark_dirty(vcpu, VCPU_EXREG_PDPTR); > mmu_reset_needed = 1; > + vcpu->arch.pdptrs_restored_oob = true; Setting pdptrs_restored_oob[*] here and _only_ clearing it on successful load_pdptrs() is not robust. Potential problems once the flag is set: 1. Userspace calls KVM_SET_SREGS{,2} without valid PDPTRs. Flag is now stale. 2. kvm_check_nested_events() VM-Exits to L1 before the flag is processed. Flag is now stale. (2) might not be problematic in practice since the "normal" load_pdptrs() should reset the flag on the next VM-Enter, but it's really, really hard to tell. E.g. what if an SMI causes an exit and _that_ non-VM-Enter reload of L2 state is the first to trip the flag? The bool is essentially an extension of KVM_REQ_GET_NESTED_STATE_PAGES, I think it makes sense to clear the flag whenever KVM_REQ_GET_NESTED_STATE_PAGES is cleared. Another thing that's not obvious is the required ordering between KVM_SET_SREGS2 and KVM_SET_NESTED_STATE. AFAICT it's not documented, but that may be PEBKAC on my end. E.g. what happens if walk_mmu == &root_mmu (L1 active in targte KVM) when SET_SREGS2 is called, and _then_ KVM_SET_NESTED_STATE is called? [*] pdptrs_from_userspace in Paolo's tree.