[PATCH 1/3] KVM: arm64: Update id_reg limit value based on per vcpu flags

[Suraj Jitindar Singh]

There are multiple features the availability of which is enabled/disabled
and tracked on a per vcpu level in vcpu->arch.flagset e.g. sve, ptrauth,
and pmu. While the vm wide value of the id regs which represent the
availability of these features is stored in the id_regs kvm struct their
value needs to be manipulated on a per vcpu basis. This is done at read
time in kvm_arm_read_id_reg().

The value of these per vcpu flags needs to be factored in when calculating
the id_reg limit value in check_features() as otherwise we can run into the
following scenario.

[ running on cpu which supports sve ]
1. AA64PFR0.SVE set in id_reg by kvm_arm_init_id_regs() (cpu supports it
   and so is set in value returned from read_sanitised_ftr_reg())
2. vcpus created without sve feature enabled
3. vmm reads AA64PFR0 and attempts to write the same value back
   (writing the same value back is allowed)
4. write fails in check_features() as limit has AA64PFR0.SVE set however it
   is not set in the value being written and although a lower value is
   allowed for this feature it is not in the mask of bits which can be
   modified and so much match exactly.

Thus add a step in check_features() to update the limit returned from
id_reg->reset() with the per vcpu features which may have been
enabled/disabled at vcpu creation time after the id_regs were initialised.
Split this update into a new function named kvm_arm_update_id_reg() so it
can be called from check_features() as well as kvm_arm_read_id_reg() to
dedup code.

Signed-off-by: Suraj Jitindar Singh <surajjs at amazon.com>
---
 arch/arm64/kvm/sys_regs.c | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
index 50d4e25f42d3..a4e662bd218b 100644
--- a/arch/arm64/kvm/sys_regs.c
+++ b/arch/arm64/kvm/sys_regs.c
@@ -42,6 +42,7 @@
  */
 
 static int set_id_reg(struct kvm_vcpu *vcpu, const struct sys_reg_desc *rd, u64 val);
+static u64 kvm_arm_update_id_reg(const struct kvm_vcpu *vcpu, u32 id, u64 val);
 static u64 kvm_arm_read_id_reg(const struct kvm_vcpu *vcpu, u32 encoding);
 static u64 sys_reg_to_index(const struct sys_reg_desc *reg);
 
@@ -1241,6 +1242,7 @@ static int arm64_check_features(struct kvm_vcpu *vcpu,
 	/* For hidden and unallocated idregs without reset, only val = 0 is allowed. */
 	if (rd->reset) {
 		limit = rd->reset(vcpu, rd);
+		limit = kvm_arm_update_id_reg(vcpu, id, limit);
 		ftr_reg = get_arm64_ftr_reg(id);
 		if (!ftr_reg)
 			return -EINVAL;
@@ -1347,10 +1349,8 @@ static u64 general_read_kvm_sanitised_reg(struct kvm_vcpu *vcpu, const struct sy
 	return read_sanitised_ftr_reg(reg_to_encoding(rd));
 }
 
-static u64 kvm_arm_read_id_reg(const struct kvm_vcpu *vcpu, u32 encoding)
+static u64 kvm_arm_update_id_reg(const struct kvm_vcpu *vcpu, u32 encoding, u64 val)
 {
-	u64 val = IDREG(vcpu->kvm, encoding);
-
 	switch (encoding) {
 	case SYS_ID_AA64PFR0_EL1:
 		if (!vcpu_has_sve(vcpu))
@@ -1402,6 +1402,13 @@ static u64 kvm_arm_read_id_reg(const struct kvm_vcpu *vcpu, u32 encoding)
 	return val;
 }
 
+static u64 kvm_arm_read_id_reg(const struct kvm_vcpu *vcpu, u32 encoding)
+{
+	u64 val = IDREG(vcpu->kvm, encoding);
+
+	return kvm_arm_update_id_reg(vcpu, encoding, val);
+}
+
 /* Read a sanitised cpufeature ID register by sys_reg_desc */
 static u64 read_id_reg(const struct kvm_vcpu *vcpu, struct sys_reg_desc const *r)
 {
-- 
2.34.1