Re: [PATCH v2 2/4] target/arm: Abstract the generic timer frequency

["Philippe Mathieu-Daudé" <philmd@redhat.com>]

On 12/3/19 1:48 PM, Andrew Jeffery wrote:
> On Tue, 3 Dec 2019, at 16:39, Philippe Mathieu-Daudé wrote:
>> On 12/3/19 5:14 AM, Andrew Jeffery wrote:
>>> Prepare for SoCs such as the ASPEED AST2600 whose firmware configures
>>> CNTFRQ to values significantly larger than the static 62.5MHz value
>>> currently derived from GTIMER_SCALE. As the OS potentially derives its
>>> timer periods from the CNTFRQ value the lack of support for running
>>> QEMUTimers at the appropriate rate leads to sticky behaviour in the
>>> guest.
>>>
>>> Substitute the GTIMER_SCALE constant with use of a helper to derive the
>>> period from gt_cntfrq stored in struct ARMCPU. Initially set gt_cntfrq
>>> to the frequency associated with GTIMER_SCALE so current behaviour is
>>> maintained.
>>>
>>> Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
>>> Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
>>> ---
>>>    target/arm/cpu.c    |  2 ++
>>>    target/arm/cpu.h    | 10 ++++++++++
>>>    target/arm/helper.c | 10 +++++++---
>>>    3 files changed, 19 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/target/arm/cpu.c b/target/arm/cpu.c
>>> index 7a4ac9339bf9..5698a74061bb 100644
>>> --- a/target/arm/cpu.c
>>> +++ b/target/arm/cpu.c
>>> @@ -974,6 +974,8 @@ static void arm_cpu_initfn(Object *obj)
>>>        if (tcg_enabled()) {
>>>            cpu->psci_version = 2; /* TCG implements PSCI 0.2 */
>>>        }
>>> +
>>> +    cpu->gt_cntfrq = NANOSECONDS_PER_SECOND / GTIMER_SCALE;
>>>    }
>>>    
>>>    static Property arm_cpu_reset_cbar_property =
>>> diff --git a/target/arm/cpu.h b/target/arm/cpu.h
>>> index 83a809d4bac4..666c03871fdf 100644
>>> --- a/target/arm/cpu.h
>>> +++ b/target/arm/cpu.h
>>> @@ -932,8 +932,18 @@ struct ARMCPU {
>>>         */
>>>        DECLARE_BITMAP(sve_vq_map, ARM_MAX_VQ);
>>>        DECLARE_BITMAP(sve_vq_init, ARM_MAX_VQ);
>>> +
>>> +    /* Generic timer counter frequency, in Hz */
>>> +    uint64_t gt_cntfrq;
>>
>> You can also explicit the unit by calling it 'gt_cntfrq_hz'.
> 
> Fair call, I'll fix that.
> 
>>
>>>    };
>>>    
>>> +static inline unsigned int gt_cntfrq_period_ns(ARMCPU *cpu)
>>> +{
>>> +    /* XXX: Could include qemu/timer.h to get NANOSECONDS_PER_SECOND? */
>>
>> Why inline this call? I doubt there is a significant performance gain.
> 
> It wasn't so much performance. It started out as a macro for a simple calculation
> because I didn't want to duplicate it across a number of places, then I wanted type
> safety for the pointer so  I switched the macro in the header to an inline function. So
> it is an evolution of the patch rather than something that came from an explicit goal
> of e.g. performance.

OK. Eventually NANOSECONDS_PER_SECOND will move to "qemu/units.h".

Should the XXX comment stay? I'm not sure, it is confusing.

Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>