Re: [PATCH v1 2/2] arch/arm: time: Add support for parsing interrupts by names

[Bertrand Marquis <Bertrand.Marquis@xxxxxxx>]

Hi Michal,

> On 9 Mar 2023, at 13:42, Michal Orzel <michal.orzel@xxxxxxx> wrote:
> 
> Hi Bertrand,
> 
> On 09/03/2023 13:19, Bertrand Marquis wrote:
>> 
>> 
>> Hi Michal,
>> 
>>> On 9 Mar 2023, at 12:35, Michal Orzel <michal.orzel@xxxxxxx> wrote:
>>> 
>>> 
>>> 
>>> On 09/03/2023 11:39, Bertrand Marquis wrote:
>>>> 
>>>> 
>>>> Hi Michal,
>>>> 
>>>>> On 9 Mar 2023, at 11:05, Michal Orzel <michal.orzel@xxxxxxx> wrote:
>>>>> 
>>>>> 
>>>>> 
>>>>> On 09/03/2023 09:02, Bertrand Marquis wrote:
>>>>>> 
>>>>>> 
>>>>>> Hi Stefano,
>>>>>> 
>>>>>>> On 7 Mar 2023, at 22:02, Stefano Stabellini <sstabellini@xxxxxxxxxx> 
>>>>>>> wrote:
>>>>>>> 
>>>>>>> On Tue, 7 Mar 2023, Bertrand Marquis wrote:
>>>>>>>>> On 7 Mar 2023, at 11:09, Andrei Cherechesu (OSS) 
>>>>>>>>> <andrei.cherechesu@xxxxxxxxxxx> wrote:
>>>>>>>>> 
>>>>>>>>> From: Andrei Cherechesu <andrei.cherechesu@xxxxxxx>
>>>>>>>>> 
>>>>>>>>> Added support for parsing the ARM generic timer interrupts DT
>>>>>>>>> node by the "interrupt-names" property, if it is available.
>>>>>>>>> 
>>>>>>>>> If not available, the usual parsing based on the expected
>>>>>>>>> IRQ order is performed.
>>>>>>>>> 
>>>>>>>>> Also added the "hyp-virt" PPI to the timer PPI list, even
>>>>>>>>> though it's currently not in use. If the "hyp-virt" PPI is
>>>>>>>>> not found, the hypervisor won't panic.
>>>>>>>>> 
>>>>>>>>> Signed-off-by: Andrei Cherechesu <andrei.cherechesu@xxxxxxx>
>>>>>>>>> ---
>>>>>>>>> xen/arch/arm/include/asm/time.h |  3 ++-
>>>>>>>>> xen/arch/arm/time.c             | 26 ++++++++++++++++++++++----
>>>>>>>>> 2 files changed, 24 insertions(+), 5 deletions(-)
>>>>>>>>> 
>>>>>>>>> diff --git a/xen/arch/arm/include/asm/time.h 
>>>>>>>>> b/xen/arch/arm/include/asm/time.h
>>>>>>>>> index 4b401c1110..49ad8c1a6d 100644
>>>>>>>>> --- a/xen/arch/arm/include/asm/time.h
>>>>>>>>> +++ b/xen/arch/arm/include/asm/time.h
>>>>>>>>> @@ -82,7 +82,8 @@ enum timer_ppi
>>>>>>>>> TIMER_PHYS_NONSECURE_PPI = 1,
>>>>>>>>> TIMER_VIRT_PPI = 2,
>>>>>>>>> TIMER_HYP_PPI = 3,
>>>>>>>>> -    MAX_TIMER_PPI = 4,
>>>>>>>>> +    TIMER_HYP_VIRT_PPI = 4,
>>>>>>>>> +    MAX_TIMER_PPI = 5,
>>>>>>>>> };
>>>>>>>>> 
>>>>>>>>> /*
>>>>>>>>> diff --git a/xen/arch/arm/time.c b/xen/arch/arm/time.c
>>>>>>>>> index 433d7be909..794da646d6 100644
>>>>>>>>> --- a/xen/arch/arm/time.c
>>>>>>>>> +++ b/xen/arch/arm/time.c
>>>>>>>>> @@ -38,6 +38,14 @@ uint32_t __read_mostly timer_dt_clock_frequency;
>>>>>>>>> 
>>>>>>>>> static unsigned int timer_irq[MAX_TIMER_PPI];
>>>>>>>>> 
>>>>>>>>> +static const char *timer_irq_names[MAX_TIMER_PPI] = {
>>>>>>>>> +    [TIMER_PHYS_SECURE_PPI] = "sec-phys",
>>>>>>>>> +    [TIMER_PHYS_NONSECURE_PPI] = "phys",
>>>>>>>>> +    [TIMER_VIRT_PPI] = "virt",
>>>>>>>>> +    [TIMER_HYP_PPI] = "hyp-phys",
>>>>>>>>> +    [TIMER_HYP_VIRT_PPI] = "hyp-virt",
>>>>>>>>> +};
>>>>>>>>> +
>>>>>>>> 
>>>>>>>> I would need some reference or a pointer to some doc to check those.
>>>>>>>> 
>>>>>>>>> unsigned int timer_get_irq(enum timer_ppi ppi)
>>>>>>>>> {
>>>>>>>>> ASSERT(ppi >= TIMER_PHYS_SECURE_PPI && ppi < MAX_TIMER_PPI);
>>>>>>>>> @@ -149,15 +157,25 @@ static void __init init_dt_xen_time(void)
>>>>>>>>> {
>>>>>>>>> int res;
>>>>>>>>> unsigned int i;
>>>>>>>>> +    bool has_names;
>>>>>>>>> +
>>>>>>>>> +    has_names = dt_property_read_bool(timer, "interrupt-names");
>>>>>>>>> 
>>>>>>>>> /* Retrieve all IRQs for the timer */
>>>>>>>>> for ( i = TIMER_PHYS_SECURE_PPI; i < MAX_TIMER_PPI; i++ )
>>>>>>>>> {
>>>>>>>>> -        res = platform_get_irq(timer, i);
>>>>>>>>> -
>>>>>>>>> -        if ( res < 0 )
>>>>>>>>> +        if ( has_names )
>>>>>>>>> +            res = platform_get_irq_byname(timer, timer_irq_names[i]);
>>>>>>>>> +        else
>>>>>>>>> +            res = platform_get_irq(timer, i);
>>>>>>>>> +
>>>>>>>>> +        if ( res > 0 )
>>>>>>>> 
>>>>>>>> The behaviour of the code is changed here compared to the current
>>>>>>>> version as res = 0 will now generate a panic.
>>>>>>>> 
>>>>>>>> Some device tree might not specify an interrupt number and just put
>>>>>>>> 0 and Xen will now panic on those systems.
>>>>>>>> As I have no idea if such systems exists and the behaviour is modified
>>>>>>>> you should justify this and mention it in the commit message or keep
>>>>>>>> the old behaviour and let 0 go through without a panic.
>>>>>>>> 
>>>>>>>> @stefano, julien any idea here ? should just keep the old behaviour ?
>>>>>>> 
>>>>>>> platform_get_irq returns 0 if the irq is 0. The irq cannot be 0 because
>>>>>>> 0 is reserved for SGIs, not PPIs. So I think it is OK to consider 0 an
>>>>>>> error.
>>>>>> 
>>>>>> Problem here is that a DTB might not specify all interrupts and just put
>>>>>> 0 for the one not used (or not available for example if you have no 
>>>>>> secure
>>>>>> world).
>>>>> Xen requires presence of EL3,EL2 and on such system, at least the 
>>>>> following timers needs to be there
>>>>> according to Arm ARM:
>>>>> - EL3 phys (if EL3 is there)
>>>> 
>>>> This might be needed by EL3 but not by Xen.
>>> Xen requires system with EL3 and if there is EL3, both Arm spec and dt 
>>> bindings requires sec-phys timer to be there.
>>> So it would be very strange to see a fake interrupt with IRQ being 0. But 
>>> if we relly want to only care about
>>> what Xen needs, then we could live with that (although it is difficult for 
>>> me to find justification for 0 there).
>>> Device trees are created per system and if system has EL3, then why forcing 
>>> 0 to be listed for sec-phys timer?
>>> 
>> 
>> Let's see that on the other angle: why should Xen check stuff that it does 
>> not need ?
> Because apart from what it needs or not, there is a matter of a failure in 
> Xen.
> Xen exposes timer IRQs to dom0 as they were taken from dtb and allowing 0 for 
> any of the timer IRQ would result
> in a Xen failure when reserving such IRQ. Xen presets SGI IRQs, meaning bits 
> 0:15 in allocated_irqs bitmap are set.
> This is why when calling vgic_reserve_virq and passing SGI always results in 
> calling a BUG().
> 
> So we have two options:
> - panic earlier with a meaningful message when IRQ is 0
> - let Xen continue and reach BUG which would be not that obvious for people 
> using but not knowing Xen

So you are saying that in the current state 0 would be ignored during scan and 
create a bug later.

If this is the case than definitely we should panic earlier with a proper 
message I agree.

Regards
Bertrand

> 
> I think first option is always better.
> 
> ~Michal