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@xxxxxxx> > > 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. _______________________________________________ kvmarm mailing list kvmarm@xxxxxxxxxxxxxxxxxxxxx https://lists.cs.columbia.edu/mailman/listinfo/kvmarm