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 +,
Wei Huang  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 

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.
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[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 
---
 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

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