Re: [PATCH v2 10/39] xen/riscv: build the target hart index via aplic_hart_field()

[Jan Beulich <jbeulich@xxxxxxxx>]

On 27.08.2026 17:20, Oleksii Kurochko wrote:
> @@ -340,27 +338,11 @@ static void cf_check aplic_set_irq_affinity(struct 
> irq_desc *desc, const cpumask
>  
>      ASSERT(spin_is_locked(&desc->lock));
>  
> -    cpu = cpuid_to_hartid(aplic_get_cpu_from_mask(mask));
> -    hhxw = imsic->group_index_bits;
> -    lhxw = imsic->hart_index_bits;
> -    /*
> -     * Although this variable is used only once in the calculation of
> -     * group_index, and it might seem that hhxs could be defined as:
> -     *   hhxs = imsic->group_index_shift - IMSIC_MMIO_PAGE_SHIFT;
> -     * and then the addition of IMSIC_MMIO_PAGE_SHIFT could be omitted
> -     * when calculating the group index.
> -     * It was done intentionally this way to follow the formula from
> -     * the AIA specification for calculating the MSI address.
> -     */
> -    hhxs = imsic->group_index_shift - IMSIC_MMIO_PAGE_SHIFT * 2;
> -    base_ppn = imsic->msi[cpu].base_addr >> IMSIC_MMIO_PAGE_SHIFT;
> -
> -    /* Update hart and EEID in the target register */
> -    group_index = (base_ppn >> (hhxs + IMSIC_MMIO_PAGE_SHIFT)) &
> -                  (BIT(hhxw, UL) - 1);
> -    value = desc->irq;

Hmm, only after sending the ack I noticed that there's no masking here, ...

> -    value |= cpu << APLIC_TARGET_HART_IDX_SHIFT;
> -    value |= group_index << (lhxw + APLIC_TARGET_HART_IDX_SHIFT);
> +    cpu = aplic_get_cpu_from_mask(mask);
> +
> +    /* Update hart index and EIID in the target register */
> +    value = MASK_INSR(aplic_hart_field(cpu), APLIC_TARGET_HART_IDX) |
> +            (desc->irq & APLIC_TARGET_EIID);

... but there is masking here. Chopping off bits doesn't look as if it can
lead to anything good. What's the deal here?

Jan