[Xen-devel] vgic initialization with maximum number of interrupt lines fails

[Lukas Jünger <lukas.juenger@xxxxxxxxxxxxxxxxxx>]

Hi all,

I use Xen in an ARMv8 simulator for research purposes.
I ran into the following problem during dom0 creation in vgic 
initialization.
Bringup fails because -EINVAL is returned by the code below.

xen/arch/arm/vgic.c in domain_vgic_init():
132     nr_spis = ROUNDUP(nr_spis, 32);
133
134     /* Limit the number of virtual SPIs supported to (1020 - 32) = 
988  */
135     if ( nr_spis > (1020 - NR_LOCAL_IRQS) )
136         return -EINVAL;

nr_spis comes from config->arch.nr_spis in xen/arch/arm/domain.c in 
arch_domain_create():
702     if ( (rc = domain_vgic_init(d, config->arch.nr_spis)) != 0 )
703         goto fail;

arch.nr_spis comes from xen/arch/arm/setup.c in start_xen():
891     dom0_cfg.arch.nr_spis = gic_number_lines() - 32;

gic_number_lines() in xen/arch/arm/gic.c:
62 unsigned int gic_number_lines(void)
63 {
64     return gic_hw_ops->info->nr_lines;
65 }

populated in xen/arch/arm/gic-v2.c in gicv2_dist_init:
354     type = readl_gicd(GICD_TYPER);
355     nr_lines = 32 * ((type & GICD_TYPE_LINES) + 1);
356     /* Only 1020 interrupts are supported */
357     nr_lines = min(1020U, nr_lines);
358     gicv2_info.nr_lines = nr_lines;

GICv2 supports up to 1020 interrupts.
In this case type will be (type & GICD_TYPE_LINES) will be 31, as the 
bits [4:0] of GICD_TYPER will be 1.
nr_lines will be 1024 and finally reduced to 1020, because of special 
purpose interrupts.
Later it will be rounded up to 1024 again and then the check will fail.
Is this expected behavior and if yes, why?

Best regards,
Lukas

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