Re: [PATCH] irqchip/gic-v3-its: Add workaround for ThunderX2 erratum #174

[Marc Zyngier]

On 03/01/18 18:13, Ganapatrao Kulkarni wrote:
> On Wed, Jan 3, 2018 at 5:06 PM, Marc Zyngier <marc.zyngier@xxxxxxx> wrote:
>> On 03/01/18 11:20, Ganapatrao Kulkarni wrote:
>>> On Wed, Jan 3, 2018 at 3:43 PM, Marc Zyngier <marc.zyngier@xxxxxxx> wrote:
>>>> On 03/01/18 09:35, Ganapatrao Kulkarni wrote:
>>>>> Hi Marc,
>>>>>
>>>>> On Wed, Jan 3, 2018 at 2:17 PM, Marc Zyngier <marc.zyngier@xxxxxxx> wrote:
>>>>>> On 03/01/18 06:32, Ganapatrao Kulkarni wrote:
>>>>>>> When an interrupt is moved across node collections on ThunderX2
>>>>>>
>>>>>> node collections?
>>>>>
>>>>> ok, i will rephrase it.
>>>>>  i was intended to say cross NUMA node collection/cpu affinity change.
>>>>>
>>>>>>
>>>>>>> multi Socket platform, an interrupt stops routed to new collection
>>>>>>> and results in loss of interrupts.
>>>>>>>
>>>>>>> Adding workaround to issue INV after MOVI for cross-node collection
>>>>>>> move to flush out the cached entry.
>>>>>>>
>>>>>>> Signed-off-by: Ganapatrao Kulkarni <ganapatrao.kulkarni@xxxxxxxxxx>
>>>>>>> ---
>>>>>>>  Documentation/arm64/silicon-errata.txt |  1 +
>>>>>>>  arch/arm64/Kconfig                     | 11 +++++++++++
>>>>>>>  drivers/irqchip/irq-gic-v3-its.c       | 24 ++++++++++++++++++++++++
>>>>>>>  3 files changed, 36 insertions(+)
>>>>>>>
>>>>>>> diff --git a/Documentation/arm64/silicon-errata.txt b/Documentation/arm64/silicon-errata.txt
>>>>>>> index fc1c884..fb27cb5 100644
>>>>>>> --- a/Documentation/arm64/silicon-errata.txt
>>>>>>> +++ b/Documentation/arm64/silicon-errata.txt
>>>>>>> @@ -63,6 +63,7 @@ stable kernels.
>>>>>>>  | Cavium         | ThunderX Core   | #27456          | CAVIUM_ERRATUM_27456        |
>>>>>>>  | Cavium         | ThunderX Core   | #30115          | CAVIUM_ERRATUM_30115        |
>>>>>>>  | Cavium         | ThunderX SMMUv2 | #27704          | N/A                         |
>>>>>>> +| Cavium         | ThunderX2 ITS   | #174            | CAVIUM_ERRATUM_174          |
>>>>>>>  | Cavium         | ThunderX2 SMMUv3| #74             | N/A                         |
>>>>>>>  | Cavium         | ThunderX2 SMMUv3| #126            | N/A                         |
>>>>>>>  |                |                 |                 |                             |
>>>>>>> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
>>>>>>> index c9a7e9e..71a7e30 100644
>>>>>>> --- a/arch/arm64/Kconfig
>>>>>>> +++ b/arch/arm64/Kconfig
>>>>>>> @@ -461,6 +461,17 @@ config ARM64_ERRATUM_843419
>>>>>>>
>>>>>>>         If unsure, say Y.
>>>>>>>
>>>>>>> +config CAVIUM_ERRATUM_174
>>>>>>> +     bool "Cavium ThunderX2 erratum 174"
>>>>>>> +     depends on NUMA
>>>>>>
>>>>>> Why? This system will be affected no matter whether NUMA is selected or not.
>>>>>
>>>>> it does not makes sense to enable on non-NUMA/single socket platforms.
>>>>> By default NUMA is enabled on ThunderX2 dual socket platforms.
>>>>
>>>> <quote>
>>>> config ARCH_THUNDER2
>>>>         bool "Cavium ThunderX2 Server Processors"
>>>>         select GPIOLIB
>>>>         help
>>>>           This enables support for Cavium's ThunderX2 CN99XX family of
>>>>           server processors.
>>>> </quote>
>>>>
>>>> Do you see any NUMA here? I can perfectly compile a kernel with both
>>>> sockets, and not using NUMA. NUMA has to do with memory, and not interrupts.
>>>
>>> ok,  i will remote it.
>>>>
>>>>>
>>>>>>
>>>>>>> +     default y
>>>>>>> +     help
>>>>>>> +       LPI stops routed to redistributors after inter node collection
>>>>>>> +       move in ITS. Enable workaround to invalidate ITS entry after
>>>>>>> +       inter-node collection move.
>>>>>>
>>>>>> That's a very terse description. Nobody knows what an LPI, a
>>>>>> redistributor or a collection is. Please explain what the erratum is in
>>>>>> layman's terms (Cavium ThunderX2 systems may loose interrupts on
>>>>>> affinity change) so that people understand whether or not they are affected.
>>>>>
>>>>> ok, i will rephrase it in next version.
>>>>>>
>>>>>>> +
>>>>>>> +       If unsure, say Y.
>>>>>>> +
>>>>>>>  config CAVIUM_ERRATUM_22375
>>>>>>>       bool "Cavium erratum 22375, 24313"
>>>>>>>       default y
>>>>>>> diff --git a/drivers/irqchip/irq-gic-v3-its.c b/drivers/irqchip/irq-gic-v3-its.c
>>>>>>> index 06f025f..d8b9c96 100644
>>>>>>> --- a/drivers/irqchip/irq-gic-v3-its.c
>>>>>>> +++ b/drivers/irqchip/irq-gic-v3-its.c
>>>>>>> @@ -46,6 +46,7 @@
>>>>>>>  #define ITS_FLAGS_CMDQ_NEEDS_FLUSHING                (1ULL << 0)
>>>>>>>  #define ITS_FLAGS_WORKAROUND_CAVIUM_22375    (1ULL << 1)
>>>>>>>  #define ITS_FLAGS_WORKAROUND_CAVIUM_23144    (1ULL << 2)
>>>>>>> +#define ITS_FLAGS_WORKAROUND_CAVIUM_174              (1ULL << 3)
>>>>>>>
>>>>>>>  #define RDIST_FLAGS_PROPBASE_NEEDS_FLUSHING  (1 << 0)
>>>>>>>
>>>>>>> @@ -1119,6 +1120,12 @@ static int its_set_affinity(struct irq_data *d, const struct cpumask *mask_val,
>>>>>>>       if (cpu != its_dev->event_map.col_map[id]) {
>>>>>>>               target_col = &its_dev->its->collections[cpu];
>>>>>>>               its_send_movi(its_dev, target_col, id);
>>>>>>> +             if (its_dev->its->flags & ITS_FLAGS_WORKAROUND_CAVIUM_174) {
>>>>>>> +                     /* Issue INV for cross node collection move. */
>>>>>>> +                     if (cpu_to_node(cpu) !=
>>>>>>> +                             cpu_to_node(its_dev->event_map.col_map[id]))
>>>>>>> +                             its_send_inv(its_dev, id);
>>>>>>> +             }
>>>>>>
>>>>>> What happens if an interrupt happens after the MOV, but before the INV?
>>>>>
>>>>> there can be drop,  if interrupt happens before INV, however, it is
>>>>> highly unlikely that we will hit the issue since MOVI and INV are
>>>>> executed back to back. this workaround fixed issue seen on couple of
>>>>> IOs.
>>>>
>>>> Really? So this doesn't fix anything, and the device may just wait
>>>> forever for the CPU to service an LPI that was never delivered. I'm
>>>> sorry, but that's not an acceptable workaround.
>>>>
>>>> I can see two solutions:
>>>> 1) you inject an interrupt after the INV as you may have lost at least one
>>>> 2) you restrict the affinity of LPIs to a single socket
>>>>
>>>> (1) will generate spurious interrupts, but will be safe. (2) will result
>>>> in an unbalanced system. Pick your poison...
>>>>
>>>> You may be able to achieve something by disabling the LPI before
>>>> performing the MOVI/INV sequence, and reenable it afterwards, but only
>>>> you can tell if this could work with your HW.
>>>
>>> thanks for the suggestion, i will try out disable/enable LPI.
>>> the sequence would be,
>>>                                   Disable LPI in rdist of cpu1
>>>                                   MOVI from cpu1 to cpu2
>>>                                   INV
>>>                                   enable LPI in  rdist of cpu1
>>
>> You cannot disable LPIs at the redistributor level. For a start, this is
>> not allowed by the architecture. It would also result in all the other
>> LPIs to be lost. What you can do is to disable the single LPI you're
>> about to move:
>>
>>         its_mask_irq(d);
>>         MOVI
>>         its_unmask_irq(d);
> 
> thanks, this also fixes the issue.
> i shall post v2 with this sequence.

Does it really? What if you get this LPI between the MOVI and the INV? I
bet it drops it on the floor.

	M.
-- 
Jazz is not dead. It just smells funny...