linux-kernel - [PATCH] KVM: X86: Reduce calls to vcpu

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Date:   Wed, 6 Sep 2023 14:24:50 +0800
From:   Hao Peng <flyingpenghao@...il.com>
To:     pbonzini@...hat.com
Cc:     kvm@...r.kernel.org, Sean Christopherson <seanjc@...gle.com>,
        linux-kernel@...r.kernel.org
Subject: [PATCH] KVM: X86: Reduce calls to vcpu_load

From: Peng Hao <flyingpeng@...cent.com>

The call of vcpu_load/put takes about 1-2us. Each
kvm_arch_vcpu_create will call vcpu_load/put
to initialize some fields of vmcs, which can be
delayed until the call of vcpu_ioctl to process
this part of the vmcs field, which can reduce calls
to vcpu_load.

Signed-off-by: Peng Hao <flyingpeng@...cent.com>
---
 arch/x86/include/asm/kvm_host.h |  1 +
 arch/x86/kvm/x86.c              | 21 ++++++++++++++++-----
 2 files changed, 17 insertions(+), 5 deletions(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 9320019708f9..2f2dcd283788 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -984,6 +984,7 @@ struct kvm_vcpu_arch {
        /* Flush the L1 Data cache for L1TF mitigation on VMENTER */
        bool l1tf_flush_l1d;

+       bool initialized;
        /* Host CPU on which VM-entry was most recently attempted */
        int last_vmentry_cpu;

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 4fd08a5e0e98..a3671a54e850 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -317,7 +317,20 @@ const struct kvm_stats_header kvm_vcpu_stats_header = {
 u64 __read_mostly host_xcr0;

 static struct kmem_cache *x86_emulator_cache;
+static int kvm_set_tsc_khz(struct kvm_vcpu *vcpu, u32 user_tsc_khz);

+static inline bool kvm_vcpu_initialized(struct kvm_vcpu *vcpu)
+{
+       return vcpu->arch.initialized;
+}
+
+static void kvm_vcpu_initial_reset(struct kvm_vcpu *vcpu)
+{
+       kvm_set_tsc_khz(vcpu, vcpu->kvm->arch.default_tsc_khz);
+       kvm_vcpu_reset(vcpu, false);
+       kvm_init_mmu(vcpu);
+       vcpu->arch.initialized = true;
+}
 /*
  * When called, it means the previous get/set msr reached an invalid msr.
  * Return true if we want to ignore/silent this failed msr access.
@@ -5647,6 +5660,9 @@ long kvm_arch_vcpu_ioctl(struct file *filp,

        vcpu_load(vcpu);

+       if (!kvm_vcpu_initialized(vcpu))
+               kvm_vcpu_initial_reset(vcpu);
+
        u.buffer = NULL;
        switch (ioctl) {
        case KVM_GET_LAPIC: {
@@ -11930,11 +11946,6 @@ int kvm_arch_vcpu_create(struct kvm_vcpu *vcpu)
        vcpu->arch.msr_platform_info = MSR_PLATFORM_INFO_CPUID_FAULT;
        kvm_xen_init_vcpu(vcpu);
        kvm_vcpu_mtrr_init(vcpu);
-       vcpu_load(vcpu);
-       kvm_set_tsc_khz(vcpu, vcpu->kvm->arch.default_tsc_khz);
-       kvm_vcpu_reset(vcpu, false);
-       kvm_init_mmu(vcpu);
-       vcpu_put(vcpu);
        return 0;

 free_guest_fpu:
--
2.31.1