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

[Julien Grall <julien.grall@xxxxxxx>]

On 22/03/2019 13:30, Lukas Jünger wrote:
> Hi all,
Hi,

> 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?
This is a known issues I haven't had time to properly fix yet. The GIC requires 
a multiple of 32 interrupts. In addition, the 2 vGIC implementation rely that 
all "existing" interrupts are fully implemented.
Some of the last lot of 32 interrupts (IRQ 992 - IRQ 1023) contains special 
purpose interrupts. The special purpose interrupts are not fully implemented. So 
it would expose a potential vulnerability issue if we were supporting up to 1020
interrupts.

This is why we limit to 992 interrupts. As Dom0 exposes the same number of 
interrupts as the host, this is an issue on platform where the GIC report 1024 
interrupts. AFAIK, those platforms are only virtual, so my idea was to limit the 
number interrupts exposed to Dom0. Something like:
dom0_cfg.arch.nr_spis = min(gic_number_lines(), 992) - 32;

Does you platform have interrupts wired above 992?

Cheers,

> Best regards,
> Lukas
>
>
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--
Julien Grall

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