Re: [PATCH v15 1/4] xen/domain: unify domain ID allocation

[Jan Beulich <jbeulich@xxxxxxxx>]

On 09.08.2025 19:08, dmkhn@xxxxxxxxx wrote:
> --- /dev/null
> +++ b/xen/common/domid.c
> @@ -0,0 +1,95 @@
> +/* SPDX-License-Identifier: GPL-2.0-only */
> +/*
> + * Domain ID allocator.
> + *
> + * Covers dom0 or late hwdom, predefined domains, post-boot domains.
> + * Excludes system domains (ID >= DOMID_FIRST_RESERVED).
> + *
> + * Copyright 2025 Ford Motor Company
> + */
> +
> +#include <xen/domain.h>
> +
> +static DEFINE_SPINLOCK(domid_lock);
> +static DECLARE_BITMAP(domid_bitmap, DOMID_FIRST_RESERVED);
> +
> +/*
> + * Allocate domain ID.
> + *
> + * @param domid Domain ID hint:
> + * - If an explicit domain ID is provided, verify its availability and use it
> + *   if ID is not used;
> + * - If DOMID_INVALID is provided, search [1..DOMID_FIRST_RESERVED-1] range,
> + *   starting from the last used ID. Implementation guarantees that two
> + *   consecutive calls will never return the same ID. ID#0 is reserved for
> + *   the first boot domain (currently, dom0) and excluded from the allocation
> + *   range.
> + * @return Valid domain ID in case of successful allocation,
> + *         DOMID_INVALID - otherwise.
> + */
> +domid_t domid_alloc(domid_t domid)
> +{
> +    static domid_t domid_last;
> +
> +    spin_lock(&domid_lock);
> +
> +    /* Exact match. */
> +    if ( domid < DOMID_FIRST_RESERVED )
> +    {
> +        if ( __test_and_set_bit(domid, domid_bitmap) )
> +            domid = DOMID_INVALID;
> +    }
> +    /*
> +     * Exhaustive search.
> +     *
> +     * Domain ID#0 is reserved for the first boot domain (e.g. control 
> domain)
> +     * and excluded from allocation.
> +     */
> +    else
> +    {
> +        domid_t num = DOMID_FIRST_RESERVED;
> +
> +        domid = find_next_zero_bit(domid_bitmap, num, domid_last + 1);
> +        if ( domid == num && domid_last != 0 )

I'm pretty sure I commented on this before, yet then maybe it was in the context
of another patch: Using == here isn't sufficient; it needs to be >=. See e.g.
cpumask_next().

Jan