[PATCH 4/6] KVM: arm64: Add a visibility bit to ignore user writes

[Oliver Upton]

We're about to ignore writes to AArch32 ID registers on AArch64-only
systems. Add a bit to indicate a register is handled as write ignore
when accessed from userspace.

Signed-off-by: Oliver Upton <oliver.upton at linux.dev>
---
 arch/arm64/kvm/sys_regs.c | 3 +++
 arch/arm64/kvm/sys_regs.h | 7 +++++++
 2 files changed, 10 insertions(+)

diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
index 26210f3a0b27..9f06c85f26b8 100644
--- a/arch/arm64/kvm/sys_regs.c
+++ b/arch/arm64/kvm/sys_regs.c
@@ -1232,6 +1232,9 @@ static int set_id_reg(struct kvm_vcpu *vcpu, const struct sys_reg_desc *rd,
 {
 	bool raz = sysreg_visible_as_raz(vcpu, rd);
 
+	if (sysreg_user_write_ignore(vcpu, rd))
+		return 0;
+
 	/* This is what we mean by invariant: you can't change it. */
 	if (val != read_id_reg(vcpu, rd, raz))
 		return -EINVAL;
diff --git a/arch/arm64/kvm/sys_regs.h b/arch/arm64/kvm/sys_regs.h
index e78b51059622..e4ebb3a379fd 100644
--- a/arch/arm64/kvm/sys_regs.h
+++ b/arch/arm64/kvm/sys_regs.h
@@ -86,6 +86,7 @@ struct sys_reg_desc {
 
 #define REG_HIDDEN		(1 << 0) /* hidden from userspace and guest */
 #define REG_RAZ			(1 << 1) /* RAZ from userspace and guest */
+#define REG_USER_WI		(1 << 2) /* WI from userspace only */
 
 static __printf(2, 3)
 inline void print_sys_reg_msg(const struct sys_reg_params *p,
@@ -157,6 +158,12 @@ static inline bool sysreg_visible_as_raz(const struct kvm_vcpu *vcpu,
 	return sysreg_visibility(vcpu, r) & REG_RAZ;
 }
 
+static inline bool sysreg_user_write_ignore(const struct kvm_vcpu *vcpu,
+					    const struct sys_reg_desc *r)
+{
+	return sysreg_visibility(vcpu, r) & REG_USER_WI;
+}
+
 static inline int cmp_sys_reg(const struct sys_reg_desc *i1,
 			      const struct sys_reg_desc *i2)
 {
-- 
2.37.1.595.g718a3a8f04-goog