Re: [PATCH v9 1/7] irqchip/gic-v3: Enable support for SGIs to act as NMIs

From: Sumit Garg
Date: Mon Aug 07 2023 - 07:27:01 EST

Hi Mark,

On Mon, 7 Aug 2023 at 15:20, Mark Rutland <mark.rutland@xxxxxxx> wrote:
>
> Hi Doug,
>
> On Thu, Jun 01, 2023 at 02:31:45PM -0700, Douglas Anderson wrote:
> > From: Sumit Garg <sumit.garg@xxxxxxxxxx>
> >
> > Add support to handle SGIs as pseudo NMIs. As SGIs or IPIs default to a
> > special flow handler: handle_percpu_devid_fasteoi_ipi(), so skip NMI
> > handler update in case of SGIs.
>
> I couldn't find handle_percpu_devid_fasteoi_ipi() in mainline, and when
> researching I found that we changed that in commit:
>
> 6abbd6988971aaa6 ("irqchip/gic, gic-v3: Make SGIs use handle_percpu_devid_irq()")
>
> ... which was in v5.11, so it looks like this is stale?

The last time I tested this patchset (v7 [1]) was with kernel
v5.9.0-rc3. So I agree with you that later
handle_percpu_devid_fasteoi_ipi() was removed and the SGI handling
flow almost became identical to PPI.

[1] https://lore.kernel.org/linux-arm-kernel/1604317487-14543-1-git-send-email-sumit.garg@xxxxxxxxxx/

>
> Since that commit, SGIs are treated the same as PPIs/EPPIs, and use
> handle_percpu_devid_irq() by default.
>
> IIUC handle_percpu_devid_irq() isn't NMI safe, and so to run in an NMI context
> those should use handle_percpu_devid_fasteoi_nmi().

True.

>
> Marc, does that sound right to you? i.e. SGI NMIs should be handled exactly the
> same as PPI NMIs, and use handle_percpu_devid_fasteoi_nmi()?
>
> I have some comments below assuming that SGI NMIs should use
> handle_percpu_devid_fasteoi_nmi().
>

This sounds fine to me.

> > Also, enable NMI support prior to gic_smp_init() as allocation of SGIs
> > as IRQs/NMIs happen as part of this routine.
>
> This bit looks fine to me.
>
> > Signed-off-by: Sumit Garg <sumit.garg@xxxxxxxxxx>
> > Reviewed-by: Masayoshi Mizuma <m.mizuma@xxxxxxxxxxxxxx>
> > Tested-by: Chen-Yu Tsai <wens@xxxxxxxx>
> > Signed-off-by: Douglas Anderson <dianders@xxxxxxxxxxxx>
> > ---
> >
> > (no changes since v1)
> >
> > drivers/irqchip/irq-gic-v3.c | 29 +++++++++++++++++++++--------
> > 1 file changed, 21 insertions(+), 8 deletions(-)
> >
> > diff --git a/drivers/irqchip/irq-gic-v3.c b/drivers/irqchip/irq-gic-v3.c
> > index 0c6c1af9a5b7..ed37e02d4c5f 100644
> > --- a/drivers/irqchip/irq-gic-v3.c
> > +++ b/drivers/irqchip/irq-gic-v3.c
> > @@ -525,6 +525,7 @@ static u32 gic_get_ppi_index(struct irq_data *d)
> > static int gic_irq_nmi_setup(struct irq_data *d)
> > {
> > struct irq_desc *desc = irq_to_desc(d->irq);
> > + u32 idx;
> >
> > if (!gic_supports_nmi())
> > return -EINVAL;
> > @@ -542,16 +543,22 @@ static int gic_irq_nmi_setup(struct irq_data *d)
> > return -EINVAL;
> >
> > /* desc lock should already be held */
> > - if (gic_irq_in_rdist(d)) {
> > - u32 idx = gic_get_ppi_index(d);
> > + switch (get_intid_range(d)) {
> > + case SGI_RANGE:
> > + break;
> > + case PPI_RANGE:
> > + case EPPI_RANGE:
> > + idx = gic_get_ppi_index(d);
> >
> > /* Setting up PPI as NMI, only switch handler for first NMI */
> > if (!refcount_inc_not_zero(&ppi_nmi_refs[idx])) {
> > refcount_set(&ppi_nmi_refs[idx], 1);
> > desc->handle_irq = handle_percpu_devid_fasteoi_nmi;
> > }
> > - } else {
> > + break;
> > + default:
> > desc->handle_irq = handle_fasteoi_nmi;
> > + break;
> > }
>
> As above, I reckon this isn't right, and we should treat all rdist interrupts
> (which are all percpu) the same.
>
> I reckon what we should be doing here is make ppi_nmi_refs cover all of the
> rdist interrupts (e.g. make that rdist_nmi_refs, add a gic_get_rdist_idx()
> helper), and then here have something like:
>
> if (gic_irq_in_rdist(d)) {
> u32 idx = gic_get_rdist_idx(d);
>
> /*
> * Setting up a percpu interrupt as NMI, only switch handler
> * for first NMI
> */
> if (!refcount_inc_not_zero(&rdist_nmi_refs[idx])) {
> refcount_set(&ppi_nmi_refs[idx], 1);
> desc->handle_irq = handle_percpu_devid_fasteoi_nmi;
> }
> }

It looks like you missed the else part here as follows for all other
interrupt types:

} else {
desc->handle_irq = handle_fasteoi_nmi;
}

Apart from that, your logic sounds good to me.

-Sumit

>
> ... as an aside, it'd be nicer if we could switch the handler at request time,
> as then we wouldn't need the refcount at all, but I couldn't see a good irqchip
> hook to hang that off, so I don't think that needs to change as a prerequisite.
>
> >
> > gic_irq_set_prio(d, GICD_INT_NMI_PRI);
> > @@ -562,6 +569,7 @@ static int gic_irq_nmi_setup(struct irq_data *d)
> > static void gic_irq_nmi_teardown(struct irq_data *d)
> > {
> > struct irq_desc *desc = irq_to_desc(d->irq);
> > + u32 idx;
> >
> > if (WARN_ON(!gic_supports_nmi()))
> > return;
> > @@ -579,14 +587,20 @@ static void gic_irq_nmi_teardown(struct irq_data *d)
> > return;
> >
> > /* desc lock should already be held */
> > - if (gic_irq_in_rdist(d)) {
> > - u32 idx = gic_get_ppi_index(d);
> > + switch (get_intid_range(d)) {
> > + case SGI_RANGE:
> > + break;
> > + case PPI_RANGE:
> > + case EPPI_RANGE:
> > + idx = gic_get_ppi_index(d);
> >
> > /* Tearing down NMI, only switch handler for last NMI */
> > if (refcount_dec_and_test(&ppi_nmi_refs[idx]))
> > desc->handle_irq = handle_percpu_devid_irq;
> > - } else {
> > + break;
> > + default:
> > desc->handle_irq = handle_fasteoi_irq;
> > + break;
> > }
>
> Same comments as for gic_irq_nmi_setup() here.
>
> >
> > gic_irq_set_prio(d, GICD_INT_DEF_PRI);
> > @@ -2001,6 +2015,7 @@ static int __init gic_init_bases(phys_addr_t dist_phys_base,
> >
> > gic_dist_init();
> > gic_cpu_init();
> > + gic_enable_nmi_support();
> > gic_smp_init();
> > gic_cpu_pm_init();
> >
> > @@ -2013,8 +2028,6 @@ static int __init gic_init_bases(phys_addr_t dist_phys_base,
> > gicv2m_init(handle, gic_data.domain);
> > }
> >
> > - gic_enable_nmi_support();
> > -
>
> This bit looks fine to me.
>
> Thanks,
> Mark.