[PATCH 1/1] KVM: arm/arm64: arch_timer: Fix TimerValue TVAL calculation in KVM

[PATCH 1/1] KVM: arm/arm64: arch_timer: Fix TimerValue TVAL calculation in KVM

[Wei Huang]

Recently the generic timer test of kvm-unit-tests failed to complete
(stalled) when a physical timer is being used. This issue is caused
by incorrect update of CNT_CVAL when TimerValue is being accessed,
introduced by 'Commit 84135d3d18da ("KVM: arm/arm64: consolidate arch
timer trap handlers")'. According to Arm ARM, the read/write behavior
of accesses to TimeValue registers is expected to be:

  * READ: TimerValue = (CompareValue – (Counter - Offset)
  * WRITE: CompareValue = ((Counter - Offset) + Sign(TimerValue)

This patch fixes the TVAL read/write code path according to the
specification.

Signed-off-by: Wei Huang <wei@redhat.com>
---
 virt/kvm/arm/arch_timer.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/virt/kvm/arm/arch_timer.c b/virt/kvm/arm/arch_timer.c
index 3417f2dbc366..d43308dc3617 100644
--- a/virt/kvm/arm/arch_timer.c
+++ b/virt/kvm/arm/arch_timer.c
@@ -812,7 +812,7 @@ static u64 kvm_arm_timer_read(struct kvm_vcpu *vcpu,
 
 	switch (treg) {
 	case TIMER_REG_TVAL:
-		val = kvm_phys_timer_read() - timer->cntvoff - timer->cnt_cval;
+		val = timer->cnt_cval - kvm_phys_timer_read() + timer->cntvoff;
 		break;
 
 	case TIMER_REG_CTL:
@@ -858,7 +858,7 @@ static void kvm_arm_timer_write(struct kvm_vcpu *vcpu,
 {
 	switch (treg) {
 	case TIMER_REG_TVAL:
-		timer->cnt_cval = val - kvm_phys_timer_read() - timer->cntvoff;
+		timer->cnt_cval = kvm_phys_timer_read() - timer->cntvoff + val;
 		break;
 
 	case TIMER_REG_CTL:
-- 
2.14.5

Re: [PATCH 1/1] KVM: arm/arm64: arch_timer: Fix TimerValue TVAL calculation in KVM

[Marc Zyngier]

On Fri, 29 Mar 2019 20:12:53 +0000,
Wei Huang <wei@redhat.com> wrote:
> 
> Recently the generic timer test of kvm-unit-tests failed to complete
> (stalled) when a physical timer is being used. This issue is caused
> by incorrect update of CNT_CVAL when TimerValue is being accessed,
> introduced by 'Commit 84135d3d18da ("KVM: arm/arm64: consolidate arch
> timer trap handlers")'. According to Arm ARM, the read/write behavior
> of accesses to TimeValue registers is expected to be:
> 
>   * READ: TimerValue = (CompareValue – (Counter - Offset)
>   * WRITE: CompareValue = ((Counter - Offset) + Sign(TimerValue)
> 
> This patch fixes the TVAL read/write code path according to the
> specification.
> 
> Signed-off-by: Wei Huang <wei@redhat.com>

Huh... Nice catch. I guess I only ran this with a VHE host, where this
code path is unused.

Thanks,

	M.

-- 
Jazz is not dead, it just smell funny.