[PATCH v3 1/4] LoongArch: KVM: Preserve auto-enabled PV features on userspace override

[Tao Cui <cui.tao@linux.dev>]

From: Tao Cui <cuitao@kylinos.cn>

When userspace (e.g., QEMU) sets PV features via KVM_SET_DEVICE_ATTR
with CPUCFG_KVM_FEATURE, it completely replaces the kernel-default
pv_features. This causes features auto-enabled by the kernel (such as
KVM_FEATURE_STEAL_TIME) to be lost if userspace doesn't explicitly
include them.

Add a pv_auto_features field to struct kvm_arch that saves the
kernel-default PV features set during kvm_vm_init_features(). When
userspace overrides pv_features, OR the saved auto features back
to preserve them.

Signed-off-by: Tao Cui <cuitao@kylinos.cn>
---
 arch/loongarch/include/asm/kvm_host.h | 2 ++
 arch/loongarch/kvm/vcpu.c             | 7 +++++++
 arch/loongarch/kvm/vm.c               | 1 +
 3 files changed, 10 insertions(+)

diff --git a/arch/loongarch/include/asm/kvm_host.h b/arch/loongarch/include/asm/kvm_host.h
index 776bc487a705..903cb67ca817 100644
--- a/arch/loongarch/include/asm/kvm_host.h
+++ b/arch/loongarch/include/asm/kvm_host.h
@@ -127,6 +127,8 @@ struct kvm_arch {
 	struct kvm_phyid_map  *phyid_map;
 	/* Enabled PV features */
 	unsigned long pv_features;
+	/* PV features auto-enabled by kernel */
+	unsigned long pv_auto_features;
 	/* Supported KVM features */
 	unsigned long kvm_features;
 
diff --git a/arch/loongarch/kvm/vcpu.c b/arch/loongarch/kvm/vcpu.c
index e28084c49e68..47a59ce9e561 100644
--- a/arch/loongarch/kvm/vcpu.c
+++ b/arch/loongarch/kvm/vcpu.c
@@ -1165,6 +1165,13 @@ static int kvm_loongarch_cpucfg_set_attr(struct kvm_vcpu *vcpu,
 		if (val & ~valid)
 			return -EINVAL;
 
+		/*
+		 * Auto-enabled features (e.g. PV TLB flush) that userspace
+		 * is not aware of are preserved so that no QEMU changes are
+		 * needed for pure kernel-internal optimisations.
+		 */
+		val = (val | kvm->arch.pv_auto_features) & valid;
+
 		/* All vCPUs need set the same PV features */
 		if ((kvm->arch.pv_features & LOONGARCH_PV_FEAT_UPDATED)
 				&& ((kvm->arch.pv_features & valid) != val))
diff --git a/arch/loongarch/kvm/vm.c b/arch/loongarch/kvm/vm.c
index 1317c718f896..8af874e0b36b 100644
--- a/arch/loongarch/kvm/vm.c
+++ b/arch/loongarch/kvm/vm.c
@@ -57,6 +57,7 @@ static void kvm_vm_init_features(struct kvm *kvm)
 		kvm->arch.kvm_features |= BIT(KVM_LOONGARCH_VM_FEAT_PV_PREEMPT);
 		kvm->arch.kvm_features |= BIT(KVM_LOONGARCH_VM_FEAT_PV_STEALTIME);
 	}
+	kvm->arch.pv_auto_features = kvm->arch.pv_features;
 }
 
 int kvm_arch_init_vm(struct kvm *kvm, unsigned long type)
-- 
2.43.0