[PATCH 3/7] KVM: arm64: Only reset vCPU-scoped feature ID regs once

Oliver Upton
2024-05-02T23:35:45Z

The general expecation with feature ID registers is that they're 'reset'
exactly once by KVM for the lifetime of a vCPU/VM, such that any
userspace changes to the CPU features / identity are honored after a
vCPU gets reset (e.g. PSCI_ON).

KVM handles what it calls VM-scoped feature ID registers correctly, but
feature ID registers local to a vCPU (CLIDR_EL1, MPIDR_EL1) get wiped
after every reset. What's especially concerning is that a
potentially-changing MPIDR_EL1 breaks MPIDR compression for indexing
mpidr_data, as the mask of useful bits to build the index could change.

This is absolutely no good. Avoid resetting vCPU feature ID registers
more than once.

Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
---
arch/arm64/include/asm/kvm_host.h | 2 ++
arch/arm64/kvm/arm.c | 5 -----
arch/arm64/kvm/sys_regs.c | 32 +++++++++++++++++++++++--------
3 files changed, 26 insertions(+), 13 deletions(-)

diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
index 9e8a496fb284..78830318c946 100644
--- a/arch/arm64/include/asm/kvm_host.h
+++ b/arch/arm64/include/asm/kvm_host.h
@@ -1275,6 +1275,8 @@ static inline bool __vcpu_has_feature(const struct kvm_arch *ka, int feature)

#define vcpu_has_feature(v, f) __vcpu_has_feature(&(v)->kvm->arch, (f))

+#define kvm_vcpu_initialized(v) vcpu_get_flag(vcpu, VCPU_INITIALIZED)
+
int kvm_trng_call(struct kvm_vcpu *vcpu);
#ifdef CONFIG_KVM
extern phys_addr_t hyp_mem_base;
diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
index c4a0a35e02c7..2116181e2315 100644
--- a/arch/arm64/kvm/arm.c
+++ b/arch/arm64/kvm/arm.c
@@ -580,11 +580,6 @@ unsigned long kvm_arch_vcpu_get_ip(struct kvm_vcpu *vcpu)
}
#endif

-static int kvm_vcpu_initialized(struct kvm_vcpu *vcpu)
-{
- return vcpu_get_flag(vcpu, VCPU_INITIALIZED);
-}
-
static void kvm_init_mpidr_data(struct kvm *kvm)
{
struct kvm_mpidr_data *data = NULL;
diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
index bb09ce4bce45..99a485062a62 100644
--- a/arch/arm64/kvm/sys_regs.c
+++ b/arch/arm64/kvm/sys_regs.c
@@ -1568,6 +1568,14 @@ static u64 read_id_reg(const struct kvm_vcpu *vcpu, const struct sys_reg_desc *r
return IDREG(vcpu->kvm, reg_to_encoding(r));
}

+static bool is_feature_id_reg(u32 encoding)
+{
+ return (sys_reg_Op0(encoding) == 3 &&
+ (sys_reg_Op1(encoding) < 2 || sys_reg_Op1(encoding) == 3) &&
+ sys_reg_CRn(encoding) == 0 &&
+ sys_reg_CRm(encoding) <= 7);
+}
+
/*
* Return true if the register's (Op0, Op1, CRn, CRm, Op2) is
* (3, 0, 0, crm, op2), where 1<=crm<8, 0<=op2<8, which is the range of ID
@@ -1580,6 +1588,11 @@ static inline bool is_vm_ftr_id_reg(u32 id)
sys_reg_CRm(id) < 8);
}

+static inline bool is_vcpu_ftr_id_reg(u32 id)
+{
+ return is_feature_id_reg(id) && !is_vm_ftr_id_reg(id);
+}
+
static inline bool is_aa32_id_reg(u32 id)
{
return (sys_reg_Op0(id) == 3 && sys_reg_Op1(id) == 0 &&
@@ -3522,6 +3535,15 @@ static void reset_vm_ftr_id_reg(struct kvm_vcpu *vcpu, const struct sys_reg_desc
IDREG(kvm, id) = reg->reset(vcpu, reg);
}

+static void reset_vcpu_ftr_id_reg(struct kvm_vcpu *vcpu,
+ const struct sys_reg_desc *reg)
+{
+ if (kvm_vcpu_initialized(vcpu))
+ return;
+
+ reg->reset(vcpu, reg);
+}
+
/**
* kvm_reset_sys_regs - sets system registers to reset value
* @vcpu: The VCPU pointer
@@ -3542,6 +3564,8 @@ void kvm_reset_sys_regs(struct kvm_vcpu *vcpu)

if (is_vm_ftr_id_reg(reg_to_encoding(r)))
reset_vm_ftr_id_reg(vcpu, r);
+ else if (is_vcpu_ftr_id_reg(reg_to_encoding(r)))
+ reset_vcpu_ftr_id_reg(vcpu, r);
else
r->reset(vcpu, r);
}
@@ -3972,14 +3996,6 @@ int kvm_arm_copy_sys_reg_indices(struct kvm_vcpu *vcpu, u64 __user *uindices)
sys_reg_CRm(r), \
sys_reg_Op2(r))

-static bool is_feature_id_reg(u32 encoding)
-{
- return (sys_reg_Op0(encoding) == 3 &&
- (sys_reg_Op1(encoding) < 2 || sys_reg_Op1(encoding) == 3) &&
- sys_reg_CRn(encoding) == 0 &&
- sys_reg_CRm(encoding) <= 7);
-}
-
int kvm_vm_ioctl_get_reg_writable_masks(struct kvm *kvm, struct reg_mask_range *range)
{
const void *zero_page = page_to_virt(ZERO_PAGE(0));