Query about IPI as NMI (pseudo-NMI) support patches

Query about IPI as NMI (pseudo-NMI) support patches

[Mukesh Ojha]

Hi Marc,

I was looking similar support mentioned in below patch series.

https://lore.kernel.org/lkml/CAFA6WYO0+LQ=mB1spCstt0cNZ0G+sZu_+Wrv6BKSeXqF5SRq4A@mail.gmail.com/#t

Wanted to check if there is chance of these patches to land in
mainline ?

Thanks,
-Mukesh

Re: Query about IPI as NMI (pseudo-NMI) support patches

[Marc Zyngier]

Hi Mukesh,

On Mon, 02 Jan 2023 16:44:59 +0000,
Mukesh Ojha <quic_mojha@quicinc.com> wrote:
> 
> Hi Marc,
> 
> I was looking similar support mentioned in below patch series.
> 
> https://lore.kernel.org/lkml/CAFA6WYO0+LQ=mB1spCstt0cNZ0G+sZu_+Wrv6BKSeXqF5SRq4A@mail.gmail.com/#t
> 
> Wanted to check if there is chance of these patches to land in
> mainline ?

I certainly have no intention to merge it as is, specially as there is
no good usage model for it other than "but think of debug!".

We have exactly *one* SGI left. If we are going to lose it over such a
feature, I'd want a description of how we are going to share it
between potential users, and how we claw some currently used SGIs
back.

Until then, this is a proof of concept, and not much else.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.

Re: Query about IPI as NMI (pseudo-NMI) support patches

[Mukesh Ojha]

Hi,

Thanks for your reply.

On 1/2/2023 10:41 PM, Marc Zyngier wrote:
> Hi Mukesh,
> 
> On Mon, 02 Jan 2023 16:44:59 +0000,
> Mukesh Ojha <quic_mojha@quicinc.com> wrote:
>>
>> Hi Marc,
>>
>> I was looking similar support mentioned in below patch series.
>>
>> https://lore.kernel.org/lkml/CAFA6WYO0+LQ=mB1spCstt0cNZ0G+sZu_+Wrv6BKSeXqF5SRq4A@mail.gmail.com/#t
>>
>> Wanted to check if there is chance of these patches to land in
>> mainline ?
> 
> I certainly have no intention to merge it as is, specially as there is
> no good usage model for it other than "but think of debug!".
> 
> We have exactly *one* SGI left. If we are going to lose it over such a
> feature, I'd want a description of how we are going to share it
> between potential users, and how we claw some currently used SGIs
> back.


But, looks like patch will fail if SGI is not available.

https://lore.kernel.org/lkml/1604317487-14543-4-git-send-email-sumit.garg@linaro.org/



set_smp_ipi_range(base_sgi, 8);

+	if (n > nr_ipi)
+		set_smp_dynamic_ipi(ipi_base + nr_ipi);
+

So, static SGI allocation still has higher priority than dynamic one.
Would you be accepting if we keep it under some CONFIG_ARM64_IPI_NMI_DEBUG ?

-Mukesh
> 
> Until then, this is a proof of concept, and not much else.
> 
> Thanks,
> 
> 	M.
> 

Re: Query about IPI as NMI (pseudo-NMI) support patches

[Marc Zyngier]

On Tue, 03 Jan 2023 16:45:04 +0000,
Mukesh Ojha <quic_mojha@quicinc.com> wrote:
> 
> Hi,
> 
> Thanks for your reply.
> 
> On 1/2/2023 10:41 PM, Marc Zyngier wrote:
> > Hi Mukesh,
> > 
> > On Mon, 02 Jan 2023 16:44:59 +0000,
> > Mukesh Ojha <quic_mojha@quicinc.com> wrote:
> >> 
> >> Hi Marc,
> >> 
> >> I was looking similar support mentioned in below patch series.
> >> 
> >> https://lore.kernel.org/lkml/CAFA6WYO0+LQ=mB1spCstt0cNZ0G+sZu_+Wrv6BKSeXqF5SRq4A@mail.gmail.com/#t
> >> 
> >> Wanted to check if there is chance of these patches to land in
> >> mainline ?
> > 
> > I certainly have no intention to merge it as is, specially as there is
> > no good usage model for it other than "but think of debug!".
> > 
> > We have exactly *one* SGI left. If we are going to lose it over such a
> > feature, I'd want a description of how we are going to share it
> > between potential users, and how we claw some currently used SGIs
> > back.
> 
> 
> But, looks like patch will fail if SGI is not available.
> 
> https://lore.kernel.org/lkml/1604317487-14543-4-git-send-email-sumit.garg@linaro.org/
> 
> 
> 
> set_smp_ipi_range(base_sgi, 8);
> 
> +	if (n > nr_ipi)
> +		set_smp_dynamic_ipi(ipi_base + nr_ipi);
> +
> 
> So, static SGI allocation still has higher priority than dynamic
> one.  Would you be accepting if we keep it under some
> CONFIG_ARM64_IPI_NMI_DEBUG ?

But why should this thing have priority over other potential features?
As I said above, there are two requirements:

- being able to share a single NMI SGI amongst multiple users

- being able to free existing SGIs in case we absolutely need an SGI
  for some other purposes

In both cases, this is about making the SGI space scale *beyond* the 8
possible interrupts that we have. This needs to be solved to get
something like this in.

And I don't think hiding this behind an obscure "debug" configuration
option that will get abused with out of tree stuff is a good move.
Quite the opposite.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.

Re: Query about IPI as NMI (pseudo-NMI) support patches

[Mukesh Ojha]

Hi Marc,

Thanks for your patience in replying queries

On 1/3/2023 11:15 PM, Marc Zyngier wrote:
> On Tue, 03 Jan 2023 16:45:04 +0000,
> Mukesh Ojha <quic_mojha@quicinc.com> wrote:
>>
>> Hi,
>>
>> Thanks for your reply.
>>
>> On 1/2/2023 10:41 PM, Marc Zyngier wrote:
>>> Hi Mukesh,
>>>
>>> On Mon, 02 Jan 2023 16:44:59 +0000,
>>> Mukesh Ojha <quic_mojha@quicinc.com> wrote:
>>>>
>>>> Hi Marc,
>>>>
>>>> I was looking similar support mentioned in below patch series.
>>>>
>>>> https://lore.kernel.org/lkml/CAFA6WYO0+LQ=mB1spCstt0cNZ0G+sZu_+Wrv6BKSeXqF5SRq4A@mail.gmail.com/#t
>>>>
>>>> Wanted to check if there is chance of these patches to land in
>>>> mainline ?
>>>
>>> I certainly have no intention to merge it as is, specially as there is
>>> no good usage model for it other than "but think of debug!".
>>>
>>> We have exactly *one* SGI left. If we are going to lose it over such a
>>> feature, I'd want a description of how we are going to share it
>>> between potential users, and how we claw some currently used SGIs
>>> back.
>>
>>
>> But, looks like patch will fail if SGI is not available.
>>
>> https://lore.kernel.org/lkml/1604317487-14543-4-git-send-email-sumit.garg@linaro.org/
>>
>>
>>
>> set_smp_ipi_range(base_sgi, 8);
>>
>> +	if (n > nr_ipi)
>> +		set_smp_dynamic_ipi(ipi_base + nr_ipi);
>> +
>>
>> So, static SGI allocation still has higher priority than dynamic
>> one.  Would you be accepting if we keep it under some
>> CONFIG_ARM64_IPI_NMI_DEBUG ?
> 
> But why should this thing have priority over other potential features?
> As I said above, there are two requirements:
> 
> - being able to share a single NMI SGI amongst multiple users
> 
> - being able to free existing SGIs in case we absolutely need an SGI
>    for some other purposes
> 
> In both cases, this is about making the SGI space scale *beyond* the 8
> possible interrupts that we have. This needs to be solved to get
> something like this in.

Agree, we have shortage of SGI's, Will try to think if we can fix this.

However, I think IPI_CPU_STOP is something which can be used as an NMI, 
As this will be used only(once) during panic()->send_smp_stop().

Can we do some special handling for IPI_CPU_STOP similar to pmu 
interrupts like request it as NMI and fallback to normal irq if not 
supported/on error ?


> 
> And I don't think hiding this behind an obscure "debug" configuration
> option that will get abused with out of tree stuff is a good move.
> Quite the opposite.
> 

Thanks, Make sense.

> Thanks,
> 
> 	M.
> 

-Mukesh

Re: Query about IPI as NMI (pseudo-NMI) support patches

[Mukesh Ojha]

Hi Marc,


On 1/4/2023 9:19 PM, Mukesh Ojha wrote:
> Hi Marc,
> 
> Thanks for your patience in replying queries
> 
> On 1/3/2023 11:15 PM, Marc Zyngier wrote:
>> On Tue, 03 Jan 2023 16:45:04 +0000,
>> Mukesh Ojha <quic_mojha@quicinc.com> wrote:
>>>
>>> Hi,
>>>
>>> Thanks for your reply.
>>>
>>> On 1/2/2023 10:41 PM, Marc Zyngier wrote:
>>>> Hi Mukesh,
>>>>
>>>> On Mon, 02 Jan 2023 16:44:59 +0000,
>>>> Mukesh Ojha <quic_mojha@quicinc.com> wrote:
>>>>>
>>>>> Hi Marc,
>>>>>
>>>>> I was looking similar support mentioned in below patch series.
>>>>>
>>>>> https://lore.kernel.org/lkml/CAFA6WYO0+LQ=mB1spCstt0cNZ0G+sZu_+Wrv6BKSeXqF5SRq4A@mail.gmail.com/#t 
>>>>>
>>>>>
>>>>> Wanted to check if there is chance of these patches to land in
>>>>> mainline ?
>>>>
>>>> I certainly have no intention to merge it as is, specially as there is
>>>> no good usage model for it other than "but think of debug!".
>>>>
>>>> We have exactly *one* SGI left. If we are going to lose it over such a
>>>> feature, I'd want a description of how we are going to share it
>>>> between potential users, and how we claw some currently used SGIs
>>>> back.
>>>
>>>
>>> But, looks like patch will fail if SGI is not available.
>>>
>>> https://lore.kernel.org/lkml/1604317487-14543-4-git-send-email-sumit.garg@linaro.org/ 
>>>
>>>
>>>
>>>
>>> set_smp_ipi_range(base_sgi, 8);
>>>
>>> +    if (n > nr_ipi)
>>> +        set_smp_dynamic_ipi(ipi_base + nr_ipi);
>>> +
>>>
>>> So, static SGI allocation still has higher priority than dynamic
>>> one.  Would you be accepting if we keep it under some
>>> CONFIG_ARM64_IPI_NMI_DEBUG ?
>>
>> But why should this thing have priority over other potential features?
>> As I said above, there are two requirements:
>>
>> - being able to share a single NMI SGI amongst multiple users
>>
>> - being able to free existing SGIs in case we absolutely need an SGI
>>    for some other purposes
>>
>> In both cases, this is about making the SGI space scale *beyond* the 8
>> possible interrupts that we have. This needs to be solved to get
>> something like this in.
> 
> Agree, we have shortage of SGI's, Will try to think if we can fix this.
> 
> However, I think IPI_CPU_STOP is something which can be used as an NMI, 
> As this will be used only(once) during panic()->send_smp_stop().
> 
> Can we do some special handling for IPI_CPU_STOP similar to pmu 
> interrupts like request it as NMI and fallback to normal irq if not 
> supported/on error ?

Can we do this ?

-Mukesh
> 
> 
>>
>> And I don't think hiding this behind an obscure "debug" configuration
>> option that will get abused with out of tree stuff is a good move.
>> Quite the opposite.
>>
> 
> Thanks, Make sense.
> 
>> Thanks,
>>
>>     M.
>>
> 
> -Mukesh