linux-kernel - Re: [PATCH] KVM: SVM: Flush Hyper-V TLB when required

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Date:   Fri, 24 Mar 2023 07:10:08 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Jeremi Piotrowski <jpiotrowski@...ux.microsoft.com>
Cc:     linux-kernel@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        kvm@...r.kernel.org, Vitaly Kuznetsov <vkuznets@...hat.com>,
        Tianyu Lan <ltykernel@...il.com>,
        Michael Kelley <mikelley@...rosoft.com>, stable@...r.kernel.org
Subject: Re: [PATCH] KVM: SVM: Flush Hyper-V TLB when required

On Fri, Mar 24, 2023, Jeremi Piotrowski wrote:
> I have the #ifdef version ready to send out, but what do you think about this:

Oh, nice!  Yeah, that works, I didn't see the stub for hyperv_flush_guest_mapping().

> @@ -3753,6 +3753,39 @@ static void svm_flush_tlb_current(struct kvm_vcpu *vcpu)
>  		svm->current_vmcb->asid_generation--;
>  }
>  
> +static void svm_flush_tlb_current(struct kvm_vcpu *vcpu)
> +{
> +	hpa_t root_tdp = vcpu->arch.mmu->root.hpa;
> +
> +	/*
> +	 * When running on Hyper-V with EnlightenedNptTlb enabled, explicitly
> +	 * flush the NPT mappings via hypercall as flushing the ASID only
> +	 * affects virtual to physical mappings, it does not invalidate guest
> +	 * physical to host physical mappings.
> +	 */
> +	if (IS_ENABLED(CONFIG_HYPERV) &&

No need for the IS_ENABLED(CONFIG_HYPERV) check here, the svm_hv_is_enlightened_tlb_enabled()
stub that's provided for CONFIG_HYPERV=n will guard this properly

	if (svm_hv_is_enlightened_tlb_enabled(vcpu) && VALID_PAGE(root_tdp))
		hyperv_flush_guest_mapping(root_tdp);

> +	    svm_hv_is_enlightened_tlb_enabled(vcpu) &&
> +	    VALID_PAGE(root_tdp))
> +		hyperv_flush_guest_mapping(root_tdp);
> +
> +	svm_flush_tlb_asid(vcpu);
> +}
> +
> +static void svm_flush_tlb_all(struct kvm_vcpu *vcpu)
> +{
> +	/*
> +	 * When running on Hyper-V with EnlightenedNptTlb enabled, remote TLB
> +	 * flushes should be routed to hv_remote_flush_tlb() without requesting
> +	 * a "regular" remote flush.  Reaching this point means either there's
> +	 * a KVM bug or a prior hv_remote_flush_tlb() call failed, both of
> +	 * which might be fatal to the guest.  Yell, but try to recover.
> +	 */
> +	if (IS_ENABLED(CONFIG_HYPERV) && WARN_ON_ONCE(svm_hv_is_enlightened_tlb_enabled(vcpu)))
> +		hv_remote_flush_tlb(vcpu->kvm);

And then

	if (WARN_ON_ONCE(svm_hv_is_enlightened_tlb_enabled(vcpu)))
		hv_remote_flush_tlb(vcpu->kvm);


> +
> +	svm_flush_tlb_asid(vcpu);
> +}
> +
>  static void svm_flush_tlb_gva(struct kvm_vcpu *vcpu, gva_t gva)
>  {
>  	struct vcpu_svm *svm = to_svm(vcpu);
> @@ -4745,10 +4778,10 @@ static struct kvm_x86_ops svm_x86_ops __initdata = {
>  	.set_rflags = svm_set_rflags,
>  	.get_if_flag = svm_get_if_flag,
>  
> -	.flush_tlb_all = svm_flush_tlb_current,
> +	.flush_tlb_all = svm_flush_tlb_all,
>  	.flush_tlb_current = svm_flush_tlb_current,
>  	.flush_tlb_gva = svm_flush_tlb_gva,
> -	.flush_tlb_guest = svm_flush_tlb_current,
> +	.flush_tlb_guest = svm_flush_tlb_asid,
>  
>  	.vcpu_pre_run = svm_vcpu_pre_run,
>  	.vcpu_run = svm_vcpu_run,
> diff --git a/arch/x86/kvm/svm/svm_onhyperv.h b/arch/x86/kvm/svm/svm_onhyperv.h
> index cff838f15db5..4c9e0d4ba3dd 100644
> --- a/arch/x86/kvm/svm/svm_onhyperv.h
> +++ b/arch/x86/kvm/svm/svm_onhyperv.h
> @@ -6,6 +6,8 @@
>  #ifndef __ARCH_X86_KVM_SVM_ONHYPERV_H__
>  #define __ARCH_X86_KVM_SVM_ONHYPERV_H__
> 
> +#include <asm/mshyperv.h>
> +
>  #if IS_ENABLED(CONFIG_HYPERV)
> 
>  #include "kvm_onhyperv.h"
> @@ -15,6 +17,14 @@ static struct kvm_x86_ops svm_x86_ops;
>  
>  int svm_hv_enable_l2_tlb_flush(struct kvm_vcpu *vcpu);
>  
> +static inline bool svm_hv_is_enlightened_tlb_enabled(struct kvm_vcpu *vcpu)
> +{
> +	struct hv_vmcb_enlightenments *hve = &to_svm(vcpu)->vmcb->control.hv_enlightenments;
> +
> +	return ms_hyperv.nested_features & HV_X64_NESTED_ENLIGHTENED_TLB &&
> +		!!hve->hv_enlightenments_control.enlightened_npt_tlb;

Uber nit, align the indentation (7 spaces instead of 1 tab):

	return ms_hyperv.nested_features & HV_X64_NESTED_ENLIGHTENED_TLB &&
	       !!hve->hv_enlightenments_control.enlightened_npt_tlb;