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

[Jeff Kubascik <jeff.kubascik@xxxxxxxxxxxxxxx>]

On 12/18/2019 9:24 AM, Julien Grall wrote:
> 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?

I'm fairly certain TimerValue register is correct. When the guest writes to the
TimerValue register, the equation below is used to convert it to a CompareValue
equivalent.

>> 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 ..."

I pulled this equation directly from the ARMv8 Reference Manual - the manual
goes into detail about the sign extension. This is referenced earlier in the
commit message.

>>
>> 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
> 

Sincerely,
Jeff Kubascik

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