linux-kernel - Re: [PATCH v3 23/37] KVM: nVMX: Add helper to handle TLB flushes on nested VM-Enter/VM-Exit

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Date:   Thu, 4 Nov 2021 17:47:53 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Lai Jiangshan <jiangshanlai+lkml@...il.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        LKML <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 v3 23/37] KVM: nVMX: Add helper to handle TLB flushes on
 nested VM-Enter/VM-Exit

On Sat, Oct 30, 2021, Lai Jiangshan wrote:
> A small comment in your proposal: I found that KVM_REQ_TLB_FLUSH_CURRENT
> and KVM_REQ_TLB_FLUSH_GUEST is to flush "current" vpid only, some special
> work needs to be added when switching mmu from L1 to L2 and vice versa:
> handle the requests before switching.

Oh, yeah, that's this snippet of my pseudo patch, but I didn't provide the
kvm_service_pending_tlb_flush_on_nested_transition() implementation so it's not
exactly obvious what I intended.  The current code handles CURRENT, but not GUEST,
the idea is to shove those into a helper that can be shared between nVMX and nSVM.

And I believe the "flush" also needs to service KVM_REQ_MMU_SYNC.  For L1=>L2 it
should be irrelevant/impossible, since L1 can only be unsync if L1 and L2 share
an MMU, but the L2=>L1 path could result in a lost sync if something, e.g. an IRQ,
prompted a nested VM-Exit before re-entering L2.

Let me know if I misunderstood your comment.  Thanks!

@@ -3361,8 +3358,7 @@ enum nvmx_vmentry_status nested_vmx_enter_non_root_mode(struct kvm_vcpu *vcpu,
        };
        u32 failed_index;

-       if (kvm_check_request(KVM_REQ_TLB_FLUSH_CURRENT, vcpu))
-               kvm_vcpu_flush_tlb_current(vcpu);
+       kvm_service_pending_tlb_flush_on_nested_transition(vcpu);

        evaluate_pending_interrupts = exec_controls_get(vmx) &
                (CPU_BASED_INTR_WINDOW_EXITING | CPU_BASED_NMI_WINDOW_EXITING);