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

[Alexandru Elisei <alexandru.elisei@arm.com>]

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.