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@xxxxxxxxxxx> 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@xxxxxxxxxxx> wrote:
> > > > Hi Marc,
> > > >
> > > > I was looking similar support mentioned in below patch series.
> > > >
> > > > https://lore.kernel.org/lkml/CAFA6WYO0+LQ=mB1spCstt0cNZ0G+sZu_+Wrv6BKSeXqF5SRq4A@xxxxxxxxxxxxxx/#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@xxxxxxxxxx/
> >
> >
> >
> > 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