linux-kernel - Re: [PATCH v3 11/34] KVM: x86: hyper-v: Use preallocated buffer in 'struct kvm_vcpu_hv' instead of on-stack 'sparse

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Message-ID: <529a8fcd87cebb24d58d403a8d5fc91f6a72d021.camel@redhat.com>
Date:   Wed, 11 May 2022 14:25:04 +0300
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>, kvm@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Michael Kelley <mikelley@...rosoft.com>,
        Siddharth Chandrasekaran <sidcha@...zon.de>,
        linux-hyperv@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 11/34] KVM: x86: hyper-v: Use preallocated buffer in
 'struct kvm_vcpu_hv' instead of on-stack 'sparse_banks'

On Thu, 2022-04-14 at 15:19 +0200, Vitaly Kuznetsov wrote:
> To make kvm_hv_flush_tlb() ready to handle L2 TLB flush requests, KVM needs
> to allow for all 64 sparse vCPU banks regardless of KVM_MAX_VCPUs as L1
> may use vCPU overcommit for L2. To avoid growing on-stack allocation, make
> 'sparse_banks' part of per-vCPU 'struct kvm_vcpu_hv' which is allocated
> dynamically.
> 
> Note: sparse_set_to_vcpu_mask() keeps using on-stack allocation as it
> won't be used to handle L2 TLB flush requests.
> 
> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
> ---
>  arch/x86/include/asm/kvm_host.h | 3 +++
>  arch/x86/kvm/hyperv.c           | 6 ++++--
>  2 files changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 058061621872..837c07e213de 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -619,6 +619,9 @@ struct kvm_vcpu_hv {
>  	} cpuid_cache;
>  
>  	struct kvm_vcpu_hv_tlb_flush_ring tlb_flush_ring[HV_NR_TLB_FLUSH_RINGS];
> +
> +	/* Preallocated buffer for handling hypercalls passing sparse vCPU set */
> +	u64 sparse_banks[64];
>  };
>  
>  /* Xen HVM per vcpu emulation context */
> diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
> index 1cef2b8f7001..e9793d36acca 100644
> --- a/arch/x86/kvm/hyperv.c
> +++ b/arch/x86/kvm/hyperv.c
> @@ -1968,6 +1968,8 @@ void kvm_hv_vcpu_flush_tlb(struct kvm_vcpu *vcpu)
>  
>  static u64 kvm_hv_flush_tlb(struct kvm_vcpu *vcpu, struct kvm_hv_hcall *hc)
>  {
> +	struct kvm_vcpu_hv *hv_vcpu = to_hv_vcpu(vcpu);
> +	u64 *sparse_banks = hv_vcpu->sparse_banks;
>  	struct kvm *kvm = vcpu->kvm;
>  	struct hv_tlb_flush_ex flush_ex;
>  	struct hv_tlb_flush flush;
> @@ -1982,7 +1984,6 @@ static u64 kvm_hv_flush_tlb(struct kvm_vcpu *vcpu, struct kvm_hv_hcall *hc)
>  	u64 __tlb_flush_entries[KVM_HV_TLB_FLUSH_RING_SIZE - 2];
>  	u64 *tlb_flush_entries;
>  	u64 valid_bank_mask;
> -	u64 sparse_banks[KVM_HV_MAX_SPARSE_VCPU_SET_BITS];
>  	struct kvm_vcpu *v;
>  	unsigned long i;
>  	bool all_cpus;
> @@ -2134,11 +2135,12 @@ static void kvm_hv_send_ipi_to_many(struct kvm *kvm, u32 vector,
>  
>  static u64 kvm_hv_send_ipi(struct kvm_vcpu *vcpu, struct kvm_hv_hcall *hc)
>  {
> +	struct kvm_vcpu_hv *hv_vcpu = to_hv_vcpu(vcpu);
> +	u64 *sparse_banks = hv_vcpu->sparse_banks;
>  	struct kvm *kvm = vcpu->kvm;
>  	struct hv_send_ipi_ex send_ipi_ex;
>  	struct hv_send_ipi send_ipi;
>  	unsigned long valid_bank_mask;
> -	u64 sparse_banks[KVM_HV_MAX_SPARSE_VCPU_SET_BITS];
>  	u32 vector;
>  	bool all_cpus;
>  

Reviewed-by: Maxim Levitsky <mlevitsk@...hat.com>

Best regards,
	Maxim Levitsky