Re: [PATCH v2] arm64: gic: increase the number of IRQ descriptors

[Shanker Donthineni]

Hi Marc,

On 1/4/23 03:14, Marc Zyngier wrote:
> External email: Use caution opening links or attachments
>
>
> On Wed, 04 Jan 2023 02:37:38 +0000,
> Shanker Donthineni <sdonthineni@xxxxxxxxxx> wrote:
> > The default value of NR_IRQS is not sufficient to support GICv4.1
> > features and ~56K LPIs. This parameter would be too small for certain
> > server platforms where it has many IO devices and is capable of
> > direct injection of vSGI and vLPI features.
> >
> > Currently, maximum of 64 + 8192 (IRQ_BITMAP_BITS) IRQ descriptors
> > are allowed. The vCPU creation fails after reaching count ~400 with
> > kvm-arm.vgic_v4_enable=1.
> >
> > This patch increases NR_IRQS to 1^19 to cover 56K LPIs and 262144
> > vSGIs (16K vPEs x 16).
> >
> > Signed-off-by: Shanker Donthineni <sdonthineni@xxxxxxxxxx>
> > ---
> > Changes since v1:
> >   -create from v6.2-rc1 and edit commit text
> >
> >   arch/arm64/include/asm/irq.h | 4 ++++
> >   1 file changed, 4 insertions(+)
> >
> > diff --git a/arch/arm64/include/asm/irq.h b/arch/arm64/include/asm/irq.h
> > index fac08e18bcd5..3fffc0b8b704 100644
> > --- a/arch/arm64/include/asm/irq.h
> > +++ b/arch/arm64/include/asm/irq.h
> > @@ -4,6 +4,10 @@
> >
> >   #ifndef __ASSEMBLER__
> >
> > +#if defined(CONFIG_ARM_GIC_V3_ITS)
> > +#define  NR_IRQS  (1 << 19)
> > +#endif
> > +
> >   #include <asm-generic/irq.h>
> >
> >   struct pt_regs;
>
> Sorry, but I don't think this is an acceptable change. This is a large
> overhead that affects *everyone*, and that will eventually be too
> small anyway with larger systems and larger interrupt spaces.
>
> A better way to address this would be to move to a more dynamic
> allocation, converting the irqdesc rb-tree into an xarray, getting rid
> of the bitmaps (the allocation bitmap and the resend one), and track
> everything in the xarray.

The actual memory allocation for IRQ descriptors is still dynamic for ARM64.
This change increases static memory for variable 'allocated_irqs' by 64KB,
feel not a noticeable overhead.

If 64KB is too high, can we change NR_IRQS to 65536.

#ifdef CONFIG_SPARSE_IRQ
# define IRQ_BITMAP_BITS	(NR_IRQS + 8196)
#else
# define IRQ_BITMAP_BITS	NR_IRQS
#endif

static DECLARE_BITMAP(allocated_irqs, IRQ_BITMAP_BITS);

For ARM64, CONFIG_SPARSE_IR is set to y.