From: Sean Christopherson <seanjc@google.com>
To: Maxim Levitsky <mlevitsk@redhat.com>
Cc: kvm@vger.kernel.org, Paolo Bonzini <pbonzini@redhat.com>,
open list <linux-kernel@vger.kernel.org>,
"open list:DOCUMENTATION" <linux-doc@vger.kernel.org>,
Wanpeng Li <wanpengli@tencent.com>,
Ingo Molnar <mingo@redhat.com>,
Vitaly Kuznetsov <vkuznets@redhat.com>,
Thomas Gleixner <tglx@linutronix.de>,
Borislav Petkov <bp@alien8.de>, Joerg Roedel <joro@8bytes.org>,
"maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)"
<x86@kernel.org>, Jim Mattson <jmattson@google.com>,
Jonathan Corbet <corbet@lwn.net>,
"H. Peter Anvin" <hpa@zytor.com>
Subject: Re: [PATCH v3 8/8] KVM: x86: avoid loading PDPTRs after migration when possible
Date: Fri, 18 Jun 2021 20:53:48 +0000 [thread overview]
Message-ID: <YM0H3Hvs8/3+twnc@google.com> (raw)
In-Reply-To: <20210607090203.133058-9-mlevitsk@redhat.com>
> 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.
next prev parent reply other threads:[~2021-06-18 20:53 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-06-07 9:01 [PATCH v3 0/8] Introduce KVM_{GET|SET}_SREGS2 and fix PDPTR migration Maxim Levitsky
2021-06-07 9:01 ` [PATCH v3 1/8] KVM: nVMX: Drop obsolete (and pointless) pdptrs_changed() check Maxim Levitsky
2021-06-07 9:01 ` [PATCH v3 2/8] KVM: nSVM: Drop pointless pdptrs_changed() check on nested transition Maxim Levitsky
2021-06-07 9:01 ` [PATCH v3 3/8] KVM: x86: Always load PDPTRs on CR3 load for SVM w/o NPT and a PAE guest Maxim Levitsky
2021-06-07 9:01 ` [PATCH v3 4/8] KVM: nSVM: refactor the CR3 reload on migration Maxim Levitsky
2021-06-07 9:02 ` [PATCH v3 5/8] KVM: nVMX: delay loading of PDPTRs to KVM_REQ_GET_NESTED_STATE_PAGES Maxim Levitsky
2021-06-07 9:02 ` [PATCH v3 6/8] KVM: x86: introduce kvm_register_clear_available Maxim Levitsky
2021-06-07 9:02 ` [PATCH v3 7/8] KVM: x86: Introduce KVM_GET_SREGS2 / KVM_SET_SREGS2 Maxim Levitsky
2021-06-10 15:06 ` Paolo Bonzini
2021-06-07 9:02 ` [PATCH v3 8/8] KVM: x86: avoid loading PDPTRs after migration when possible Maxim Levitsky
2021-06-18 20:53 ` Sean Christopherson [this message]
2021-06-19 7:03 ` Paolo Bonzini
2021-06-20 22:25 ` Maxim Levitsky
2021-06-10 15:06 ` [PATCH v3 0/8] Introduce KVM_{GET|SET}_SREGS2 and fix PDPTR migration Paolo Bonzini
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