linux-kernel - [PATCH RFC 08/19] KVM: x86: hyper-v: Use preallocated buffer in 'struct kvm_vcpu_hv' instead of on-stack 'sparse

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Date:   Mon,  7 Mar 2022 15:50:12 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     kvm@...r.kernel.org
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        linux-hyperv@...r.kernel.org,
        Siddharth Chandrasekaran <sidcha@...zon.de>,
        linux-kernel@...r.kernel.org
Subject: [PATCH RFC 08/19] KVM: x86: hyper-v: Use preallocated buffer in 'struct kvm_vcpu_hv' instead of on-stack 'sparse_banks'

To make kvm_hv_flush_tlb() ready to handle Direct TLB flush request
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 Direct 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 750ac4055d0c..1fce8232a2e9 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -615,6 +615,9 @@ struct kvm_vcpu_hv {
 
 	/* Two rings for regular Hyper-V TLB flush and Direct TLB flush */
 	struct kvm_vcpu_hv_tlbflush_ring tlb_flush_ring[2];
+
+	/* 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 9dfc122d5eca..b88e44a126b8 100644
--- a/arch/x86/kvm/hyperv.c
+++ b/arch/x86/kvm/hyperv.c
@@ -1969,13 +1969,14 @@ 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;
 	DECLARE_BITMAP(vcpu_mask, KVM_MAX_VCPUS);
 	u64 entries[KVM_HV_TLB_FLUSH_RING_SIZE - 2];
 	u64 valid_bank_mask;
-	u64 sparse_banks[KVM_HV_MAX_SPARSE_VCPU_SET_BITS];
 	struct kvm_vcpu *v;
 	unsigned long i;
 	bool all_cpus, all_addr;
@@ -2127,11 +2128,12 @@ 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;
 
-- 
2.35.1