[PATCH] KVM: arm64: Treat emulated TVAL TimerValue as a signed 32-bit integer

[Alexandru Elisei]

According to the ARM ARM, registers CNT{P,V}_TVAL_EL0 have bits [63:32]
RES0 [1]. When reading the register, the value is truncated to the least
significant 32 bits [2], and on writes, TimerValue is treated as a signed
32-bit integer [1, 2].

When the guest behaves correctly and writes 32-bit values, treating TVAL
as an unsigned 64 bit register works as expected. However, things start
to break down when the guest writes larger values, because
(u64)0x1_ffff_ffff = 8589934591. but (s32)0x1_ffff_ffff = -1, and the
former will cause the timer interrupt to be asserted in the future, but
the latter will cause it to be asserted now.  Let's treat TVAL as a
signed 32-bit register on writes, to match the behaviour described in
the architecture, and the behaviour experimentally exhibited by the
virtual timer on a non-vhe host.

[1] Arm DDI 0487E.a, section D13.8.18
[2] Arm DDI 0487E.a, section D11.2.4

Signed-off-by: Alexandru Elisei <alexandru.elisei@xxxxxxx>
---
 include/kvm/arm_arch_timer.h | 2 ++
 virt/kvm/arm/arch_timer.c    | 3 ++-
 2 files changed, 4 insertions(+), 1 deletion(-)

diff --git a/include/kvm/arm_arch_timer.h b/include/kvm/arm_arch_timer.h
index d120e6c323e7..be912176b7a3 100644
--- a/include/kvm/arm_arch_timer.h
+++ b/include/kvm/arm_arch_timer.h
@@ -10,6 +10,8 @@
 #include <linux/clocksource.h>
 #include <linux/hrtimer.h>
 
+#define ARCH_TIMER_TVAL_MASK	((1ULL << 32) - 1)
+
 enum kvm_arch_timers {
 	TIMER_PTIMER,
 	TIMER_VTIMER,
diff --git a/virt/kvm/arm/arch_timer.c b/virt/kvm/arm/arch_timer.c
index f182b2380345..5d40f17f7024 100644
--- a/virt/kvm/arm/arch_timer.c
+++ b/virt/kvm/arm/arch_timer.c
@@ -805,6 +805,7 @@ static u64 kvm_arm_timer_read(struct kvm_vcpu *vcpu,
 	switch (treg) {
 	case TIMER_REG_TVAL:
 		val = timer->cnt_cval - kvm_phys_timer_read() + timer->cntvoff;
+		val &= ARCH_TIMER_TVAL_MASK;
 		break;
 
 	case TIMER_REG_CTL:
@@ -850,7 +851,7 @@ static void kvm_arm_timer_write(struct kvm_vcpu *vcpu,
 {
 	switch (treg) {
 	case TIMER_REG_TVAL:
-		timer->cnt_cval = kvm_phys_timer_read() - timer->cntvoff + val;
+		timer->cnt_cval = kvm_phys_timer_read() - timer->cntvoff + (s32)val;
 		break;
 
 	case TIMER_REG_CTL:
-- 
2.20.1