linux-kernel - Re: "KVM: x86/mmu: Overhaul TDP MMU zapping and flushing" breaks SVM on Hyper-V

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Date:   Mon, 13 Feb 2023 19:26:41 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Jeremi Piotrowski <jpiotrowski@...ux.microsoft.com>,
        Sean Christopherson <seanjc@...gle.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Tianyu Lan <ltykernel@...il.com>,
        "Michael Kelley (LINUX)" <mikelley@...rosoft.com>
Subject: Re: "KVM: x86/mmu: Overhaul TDP MMU zapping and flushing" breaks SVM
 on Hyper-V

On 2/13/23 19:05, Jeremi Piotrowski wrote:
> So I looked at the ftrace (all kvm&kvmmu events + hyperv_nested_*
> events) I see the following: With tdp_mmu=0: kvm_exit sequence of
> kvm_mmu_prepare_zap_page hyperv_nested_flush_guest_mapping (always
> follows every sequence of kvm_mmu_prepare_zap_page) kvm_entry
> 
> With tdp_mmu=1 I see: kvm_mmu_prepare_zap_page and
> kvm_tdp_mmu_spte_changed events from a kworker context, but they are
> not followed by hyperv_nested_flush_guest_mapping. The only
> hyperv_nested_flush_guest_mapping events I see happen from the qemu
> process context.
> 
> Also the number of flush hypercalls is significantly lower: a 7second
> sequence through OVMF with tdp_mmu=0 produces ~270 flush hypercalls.
> In the traces with tdp_mmu=1 I now see max 3.
> 
> So this might be easier to diagnose than I thought: the
> HvCallFlushGuestPhysicalAddressSpace calls are missing now.

Can you check if KVM is reusing a nCR3 value?

If so, perhaps you can just add 
hyperv_flush_guest_mapping(__pa(root->spt), NULL) after 
kvm_tdp_mmu_get_vcpu_root_hpa's call to tdp_mmu_alloc_sp()?

Paolo