Re: [Xen-devel] [PATCH v3 2/2] xen/arm: sign extend writes to TimerValue

[Julien Grall <julien@xxxxxxx>]

Hi Jeff,

On 11/12/2019 21:13, Jeff Kubascik wrote:
> Per the ARMv8 Reference Manual (ARM DDI 0487E.a), section D11.2.4
> specifies that the values in the TimerValue view of the timers are
> signed in standard two's complement form. When writing to the TimerValue
Do you mean CompareValue register instead of TimerValue register?

> register, it should be signed extended as described by the equation
>
>     CompareValue = (Counter[63:0] + SignExtend(TimerValue))[63:0]
This explains the signed part, but it does not explain why the 32-bit 
case. So I would mention that TimerValue is a 32-bit signed integer.
Maybe saying "are 32-bit signed in standard ..."

> Signed-off-by: Jeff Kubascik <jeff.kubascik@xxxxxxxxxxxxxxx>
> ---
>   xen/arch/arm/vtimer.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/xen/arch/arm/vtimer.c b/xen/arch/arm/vtimer.c
> index 21b98ec20a..872181d9b6 100644
> --- a/xen/arch/arm/vtimer.c
> +++ b/xen/arch/arm/vtimer.c
> @@ -211,7 +211,7 @@ static bool vtimer_cntp_tval(struct cpu_user_regs *regs, 
> uint32_t *r,
>       }
>       else
>       {
> -        v->arch.phys_timer.cval = cntpct + *r;
> +        v->arch.phys_timer.cval = cntpct + (uint64_t)(int32_t)*r;
>           if ( v->arch.phys_timer.ctl & CNTx_CTL_ENABLE )
>           {
>               v->arch.phys_timer.ctl &= ~CNTx_CTL_PENDING;
Cheers,

--
Julien Grall

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