Re: [PATCH 02/14] KVM: x86: hyper-v: Move Hyper-V partition assist page out of Hyper-V emulation context

[Maxim Levitsky]

On Wed, 2023-10-25 at 17:23 +0200, Vitaly Kuznetsov wrote:
> Hyper-V partition assist page is used when KVM runs on top of Hyper-V and
> is not used for Windows/Hyper-V guests on KVM, this means that 'hv_pa_pg'
> placement in 'struct kvm_hv' is unfortunate. As a preparation to making
> Hyper-V emulation optional, move 'hv_pa_pg' to 'struct kvm_arch' and put it
> under CONFIG_HYPERV.
> 
> While on it, introduce hv_get_partition_assist_page() helper to allocate
> partition assist page. Move the comment explaining why we use a single page
> for all vCPUs from VMX and expand it a bit.
> 
> No functional change intended.
> 
> Signed-off-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
> ---
>  arch/x86/include/asm/kvm_host.h |  2 +-
>  arch/x86/kvm/kvm_onhyperv.h     | 20 ++++++++++++++++++++
>  arch/x86/kvm/svm/svm_onhyperv.c | 10 +++-------
>  arch/x86/kvm/vmx/vmx.c          | 14 +++-----------
>  arch/x86/kvm/x86.c              |  4 +++-
>  5 files changed, 30 insertions(+), 20 deletions(-)
> 
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index d107516b4591..7fb2810f4573 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -1125,7 +1125,6 @@ struct kvm_hv {
>  	 */
>  	unsigned int synic_auto_eoi_used;
>  
> -	struct hv_partition_assist_pg *hv_pa_pg;
>  	struct kvm_hv_syndbg hv_syndbg;
>  };
>  
> @@ -1447,6 +1446,7 @@ struct kvm_arch {
>  #if IS_ENABLED(CONFIG_HYPERV)
>  	hpa_t	hv_root_tdp;
>  	spinlock_t hv_root_tdp_lock;
> +	struct hv_partition_assist_pg *hv_pa_pg;
>  #endif
>  	/*
>  	 * VM-scope maximum vCPU ID. Used to determine the size of structures
> diff --git a/arch/x86/kvm/kvm_onhyperv.h b/arch/x86/kvm/kvm_onhyperv.h
> index f9ca3e7432b2..eefab3dc8498 100644
> --- a/arch/x86/kvm/kvm_onhyperv.h
> +++ b/arch/x86/kvm/kvm_onhyperv.h
> @@ -10,6 +10,26 @@
>  int hv_flush_remote_tlbs_range(struct kvm *kvm, gfn_t gfn, gfn_t nr_pages);
>  int hv_flush_remote_tlbs(struct kvm *kvm);
>  void hv_track_root_tdp(struct kvm_vcpu *vcpu, hpa_t root_tdp);
> +static inline hpa_t hv_get_partition_assist_page(struct kvm_vcpu *vcpu)
> +{
> +	/*
> +	 * Partition assist page is something which Hyper-V running in L0
> +	 * requires from KVM running in L1 before direct TLB flush for L2
> +	 * guests can be enabled. KVM doesn't currently use the page but to
> +	 * comply with TLFS it still needs to be allocated. For now, this
> +	 * is a single page shared among all vCPUs.
Perfect!

> +	 */
> +	struct hv_partition_assist_pg **p_hv_pa_pg =
> +		&vcpu->kvm->arch.hv_pa_pg;
> +
> +	if (!*p_hv_pa_pg)
> +		*p_hv_pa_pg = kzalloc(PAGE_SIZE, GFP_KERNEL_ACCOUNT);
> +
> +	if (!*p_hv_pa_pg)
> +		return INVALID_PAGE;
> +
> +	return __pa(*p_hv_pa_pg);
> +}
>  #else /* !CONFIG_HYPERV */
>  static inline int hv_flush_remote_tlbs(struct kvm *kvm)
>  {
> diff --git a/arch/x86/kvm/svm/svm_onhyperv.c b/arch/x86/kvm/svm/svm_onhyperv.c
> index 7af8422d3382..3971b3ea5d04 100644
> --- a/arch/x86/kvm/svm/svm_onhyperv.c
> +++ b/arch/x86/kvm/svm/svm_onhyperv.c
> @@ -18,18 +18,14 @@
>  int svm_hv_enable_l2_tlb_flush(struct kvm_vcpu *vcpu)
>  {
>  	struct hv_vmcb_enlightenments *hve;
> -	struct hv_partition_assist_pg **p_hv_pa_pg =
> -			&to_kvm_hv(vcpu->kvm)->hv_pa_pg;
> +	hpa_t partition_assist_page = hv_get_partition_assist_page(vcpu);
>  
> -	if (!*p_hv_pa_pg)
> -		*p_hv_pa_pg = kzalloc(PAGE_SIZE, GFP_KERNEL);
> -
> -	if (!*p_hv_pa_pg)
> +	if (partition_assist_page == INVALID_PAGE)
>  		return -ENOMEM;
>  
>  	hve = &to_svm(vcpu)->vmcb->control.hv_enlightenments;
>  
> -	hve->partition_assist_page = __pa(*p_hv_pa_pg);
> +	hve->partition_assist_page = partition_assist_page;
>  	hve->hv_vm_id = (unsigned long)vcpu->kvm;
>  	if (!hve->hv_enlightenments_control.nested_flush_hypercall) {
>  		hve->hv_enlightenments_control.nested_flush_hypercall = 1;
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index be20a60047b1..cb4591405f14 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -523,22 +523,14 @@ module_param(enlightened_vmcs, bool, 0444);
>  static int hv_enable_l2_tlb_flush(struct kvm_vcpu *vcpu)
>  {
>  	struct hv_enlightened_vmcs *evmcs;
> -	struct hv_partition_assist_pg **p_hv_pa_pg =
> -			&to_kvm_hv(vcpu->kvm)->hv_pa_pg;
> -	/*
> -	 * Synthetic VM-Exit is not enabled in current code and so All
> -	 * evmcs in singe VM shares same assist page.
> -	 */
> -	if (!*p_hv_pa_pg)
> -		*p_hv_pa_pg = kzalloc(PAGE_SIZE, GFP_KERNEL_ACCOUNT);
> +	hpa_t partition_assist_page = hv_get_partition_assist_page(vcpu);
>  
> -	if (!*p_hv_pa_pg)
> +	if (partition_assist_page == INVALID_PAGE)
>  		return -ENOMEM;
>  
>  	evmcs = (struct hv_enlightened_vmcs *)to_vmx(vcpu)->loaded_vmcs->vmcs;
>  
> -	evmcs->partition_assist_page =
> -		__pa(*p_hv_pa_pg);
> +	evmcs->partition_assist_page = partition_assist_page;
>  	evmcs->hv_vm_id = (unsigned long)vcpu->kvm;
>  	evmcs->hv_enlightenments_control.nested_flush_hypercall = 1;
>  
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index d632931fa545..cc2524598368 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -12425,7 +12425,9 @@ void kvm_arch_sched_in(struct kvm_vcpu *vcpu, int cpu)
>  
>  void kvm_arch_free_vm(struct kvm *kvm)
>  {
> -	kfree(to_kvm_hv(kvm)->hv_pa_pg);
> +#if IS_ENABLED(CONFIG_HYPERV)
> +	kfree(kvm->arch.hv_pa_pg);
> +#endif
>  	__kvm_arch_free_vm(kvm);
>  }
>  


Reviewed-by: Maxim Levitsky <mlevitsk@xxxxxxxxxx>

Best regards,
	Maxim Levitsky