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

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Date:   Mon, 27 Jan 2020 11:07:52 +0000
From:   Marc Zyngier <maz@...nel.org>
To:     Alexandru Elisei <alexandru.elisei@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        linux-kernel@...r.kernel.org, james.morse@....com,
        julien.thierry.kdev@...il.com, suzuki.poulose@....com
Subject: Re: [PATCH] KVM: arm64: Treat emulated TVAL TimerValue as a signed
 32-bit integer

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@....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...