Re: [PATCH 23/23] KVM: arm64: Treat emulated TVAL TimerValue as a signed 32-bit integer

Alexandru Elisei
2020-01-30T14:12:02Z

Hi,

On 1/30/20 1:25 PM, Marc Zyngier wrote:
> From: Alexandru Elisei <alexandru.elisei@arm.com>
>
> 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>
> [maz: replaced the read-side mask with lower_32_bits]
> Signed-off-by: Marc Zyngier <maz@kernel.org>
> Fixes: 8fa761624871 ("KVM: arm/arm64: arch_timer: Fix CNTP_TVAL calculation")
> Link: https://lore.kernel.org/r/20200127103652.2326-1-alexandru.elisei@arm.com
> ---
> virt/kvm/arm/arch_timer.c | 3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/virt/kvm/arm/arch_timer.c b/virt/kvm/arm/arch_timer.c
> index f182b2380345..c6c2a9dde00c 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 &= lower_32_bits(val);

This is correct, but how about making it val = lower_32_bits(val) for more
clarity? Apologies for not spotting it earlier :(

Thanks,
Alex
> 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;
>