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

[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@arm.com>
---
 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

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

[Marc Zyngier]

Hi Alexandru,

On 2020-01-27 10:36, Alexandru Elisei wrote:
> 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@arm.com>

Huhuh... Nice catch!

Fixes: 8fa761624871 ("KVM: arm/arm64: arch_timer: Fix CNTP_TVAL 
calculation")

(how many times are we doing to fix this???)

> ---
>  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;

nit: Do we really need this mask? I'd rather see it written as

                 val = lower_32_bits(val);


>  		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:

Otherwise, looks good to me. If you're OK with the above change, I'll
take it as a fix.

Thanks,

         M.
-- 
Jazz is not dead. It just smells funny...

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

[Alexandru Elisei]

Hi,

On 1/27/20 11:07 AM, Marc Zyngier wrote:
> Hi Alexandru,
>
> On 2020-01-27 10:36, Alexandru Elisei wrote:
>> 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@arm.com>
>
> Huhuh... Nice catch!
>
> Fixes: 8fa761624871 ("KVM: arm/arm64: arch_timer: Fix CNTP_TVAL calculation")
>
> (how many times are we doing to fix this???)
>
>> ---
>>  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;
>
> nit: Do we really need this mask? I'd rather see it written as
>
>                 val = lower_32_bits(val);

I didn't really like using the mask either, but I couldn't think of anything
better. This looks very good.

>
>
>>          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:
>
> Otherwise, looks good to me. If you're OK with the above change, I'll
> take it as a fix.

Yes, I'm very much OK with the change.

Thanks,
Alex
>
> Thanks,
>
>         M.