[PATCH v4 01/26] KVM: x86: Drop kvm_user_return_msr_cpu_online()

[isaku . yamahata]

From: Sean Christopherson <seanjc@xxxxxxxxxx>

KVM/X86 uses user return notifier to switch MSR for guest or user space.
Snapshot host values on CPU online, change MSR values for guest, and
restore them on returning to user space.  The current code abuses
kvm_arch_hardware_enable() which is called on kvm module initialization or
CPU online.

Remove such the abuse of kvm_arch_hardware_enable() by capturing the host
value on the first change of the MSR value to guest VM instead of CPU
online.

Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
Signed-off-by: Isaku Yamahata <isaku.yamahata@xxxxxxxxx>
Reviewed-by: Chao Gao <chao.gao@xxxxxxxxx>
Reviewed-by: Yuan Yao <yuan.yao@xxxxxxxxx>
---
 arch/x86/kvm/x86.c | 19 ++++++++++++++-----
 1 file changed, 14 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 205ebdc2b11b..73dccc952dd1 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -196,6 +196,7 @@ module_param(eager_page_split, bool, 0644);
 
 struct kvm_user_return_msrs {
 	struct user_return_notifier urn;
+	bool initialized;
 	bool registered;
 	struct kvm_user_return_msr_values {
 		u64 host;
@@ -409,18 +410,20 @@ int kvm_find_user_return_msr(u32 msr)
 }
 EXPORT_SYMBOL_GPL(kvm_find_user_return_msr);
 
-static void kvm_user_return_msr_cpu_online(void)
+static void kvm_user_return_msr_init_cpu(struct kvm_user_return_msrs *msrs)
 {
-	unsigned int cpu = smp_processor_id();
-	struct kvm_user_return_msrs *msrs = per_cpu_ptr(user_return_msrs, cpu);
 	u64 value;
 	int i;
 
+	if (msrs->initialized)
+		return;
+
 	for (i = 0; i < kvm_nr_uret_msrs; ++i) {
 		rdmsrl_safe(kvm_uret_msrs_list[i], &value);
 		msrs->values[i].host = value;
 		msrs->values[i].curr = value;
 	}
+	msrs->initialized = true;
 }
 
 int kvm_set_user_return_msr(unsigned slot, u64 value, u64 mask)
@@ -429,6 +432,8 @@ int kvm_set_user_return_msr(unsigned slot, u64 value, u64 mask)
 	struct kvm_user_return_msrs *msrs = per_cpu_ptr(user_return_msrs, cpu);
 	int err;
 
+	kvm_user_return_msr_init_cpu(msrs);
+
 	value = (value & mask) | (msrs->values[slot].host & ~mask);
 	if (value == msrs->values[slot].curr)
 		return 0;
@@ -9212,7 +9217,12 @@ int kvm_arch_init(void *opaque)
 		return -ENOMEM;
 	}
 
-	user_return_msrs = alloc_percpu(struct kvm_user_return_msrs);
+	/*
+	 * __GFP_ZERO to ensure user_return_msrs.initialized = false.
+	 * See kvm_user_return_msr_init_cpu().
+	 */
+	user_return_msrs = alloc_percpu_gfp(struct kvm_user_return_msrs,
+					    GFP_KERNEL | __GFP_ZERO);
 	if (!user_return_msrs) {
 		printk(KERN_ERR "kvm: failed to allocate percpu kvm_user_return_msrs\n");
 		r = -ENOMEM;
@@ -11836,7 +11846,6 @@ int kvm_arch_hardware_enable(void)
 	u64 max_tsc = 0;
 	bool stable, backwards_tsc = false;
 
-	kvm_user_return_msr_cpu_online();
 	ret = static_call(kvm_x86_hardware_enable)();
 	if (ret != 0)
 		return ret;
-- 
2.25.1