linux-kernel - Re: [PATCH v2 31/32] KVM: nVMX: Don't flush TLB on nested VM transition with EPT enabled

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Message-ID: <7661081d-6276-6176-dbbb-700aeec656b8@redhat.com>
Date:   Wed, 18 Mar 2020 18:38:56 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Ben Gardon <bgardon@...gle.com>,
        Junaid Shahid <junaids@...gle.com>,
        Liran Alon <liran.alon@...cle.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        John Haxby <john.haxby@...cle.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Tom Lendacky <thomas.lendacky@....com>
Subject: Re: [PATCH v2 31/32] KVM: nVMX: Don't flush TLB on nested VM
 transition with EPT enabled

On 18/03/20 18:26, Sean Christopherson wrote:
>>>
>>> 	if (!nested_ept)
>>> 		kvm_mmu_new_cr3(vcpu, cr3, enable_ept ||
>>> 					   nested_cpu_has_vpid(vmcs12));
>>
>> ... which is exactly nested_has_guest_tlb_tag(vcpu).  Well, not exactly
>> but it's a bug in your code above. :)
>
> I don't think it's a bug, it's intentionally different.  When enable_ept=0,
> nested_has_guest_tlb_tag() returns true if and only if L1 has enabled VPID
> for L2 *and* L2 has been assigned a unique VPID by L0.
> 
> For sync purposes, whether or not L2 has been assigned a unique VPID is
> irrelevant.  L0 needs to invalidate TLB entries to prevent resuing L1's
> entries (assuming L1 has been assigned a VPID), but L0 doesn't need to sync
> SPTEs because L2 doesn't expect them to be refreshed.
                ^^
                L1

Yes you're right.  So I would say keep your code, but we can simplify
the comment.  Something like:

/*
 * We can skip the TLB flush if we have EPT enabled (because...)  and
 * also if L1 is using VPID, because then it doesn't expect SPTEs for L2
 * to be refreshed.
 *
 * This is almost the same as nested_has_guest_tlb_tag(vcpu), but here
 * we don't care if L2 has been assigned a unique VPID; L1 doesn't know,
 * and will nevertheless do INVVPID to avoid reuse of stale page
 * table entries.
 */

Nevertheless it's scary in that this is a potential attack vector for
reusing stale L0 SPTEs, so we should make sure it's all properly commented.

Thanks,

Paolo

>> It completely makes sense to use that as the third argument, and while a
>> comment is still needed it will be much smaller.
> Ya, agreed.
>