Re: [PATCH v6 1/2] xen/domain: unify domain ID allocation

[dmkhn@proton.me]

On Fri, May 16, 2025 at 08:43:35AM +0000, Teddy Astie wrote:
> Hello,
> 
> Le 16/05/2025 à 04:06, dmkhn@proton.me a écrit :
> > From: Denis Mukhin <dmukhin@ford.com>
> >
> > Currently, hypervisor code has two different non-system domain ID allocation
> > implementations:
> >
> >    (a) Sequential IDs allocation in dom0less Arm code based on max_init_domid;
> >
> >    (b) Sequential IDs allocation in XEN_DOMCTL_createdomain; does not use
> >        max_init_domid (both Arm and x86).
> >
> > It makes sense to have a common helper code for such task across architectures
> > (Arm and x86) and between dom0less / toolstack domU allocation.
> >
> > Wrap the domain ID allocation as an arch-independent function domid_alloc() in
> > common/domain.c based on rangeset.
> >
> > Allocation algorithm:
> > - If an explicit domain ID is provided, verify its availability and
> >    use it if ID is not used;
> > - Otherwise, perform an exhaustive search starting from the end of the used
> >    domain ID range. domid_alloc() guarantees that two subsequent calls will
> >    result in different IDs allocation.
> >
> > Initialize the domain IDs rangeset from the new domid_init() which is called
> > from arch setup code.
> >
> > Also, remove is_free_domid() helper as it is not needed now.
> >
> > No functional change intended.
> >
> > Signed-off-by: Denis Mukhin <dmukhin@ford.com>
> > ---
> > Changes since v5:
> > - rebased
> > ---
> >   xen/arch/arm/domain_build.c             | 17 ++++--
> >   xen/arch/arm/setup.c                    |  2 +
> >   xen/arch/x86/setup.c                    | 13 +++--
> >   xen/common/device-tree/dom0less-build.c | 10 ++--
> >   xen/common/domain.c                     | 70 +++++++++++++++++++++++++
> >   xen/common/domctl.c                     | 41 ++-------------
> >   xen/include/xen/domain.h                |  4 ++
> >   7 files changed, 107 insertions(+), 50 deletions(-)
> >
> > diff --git a/xen/arch/arm/domain_build.c b/xen/arch/arm/domain_build.c
> > index b189a7cfae..e9d563c269 100644
> > --- a/xen/arch/arm/domain_build.c
> > +++ b/xen/arch/arm/domain_build.c
> > @@ -2010,6 +2010,7 @@ void __init create_dom0(void)
> >           .grant_opts = XEN_DOMCTL_GRANT_version(opt_gnttab_max_version),
> >       };
> >       unsigned int flags = CDF_privileged | CDF_hardware;
> > +    domid_t domid;
> >       int rc;
> >
> >       /* The vGIC for DOM0 is exactly emulating the hardware GIC */
> > @@ -2034,19 +2035,25 @@ void __init create_dom0(void)
> >       if ( !llc_coloring_enabled )
> >           flags |= CDF_directmap;
> >
> > -    dom0 = domain_create(0, &dom0_cfg, flags);
> > +    domid = domid_alloc(0);
> > +    if ( domid == DOMID_INVALID )
> > +        panic("Error allocating domain ID 0\n");
> > +
> > +    dom0 = domain_create(domid, &dom0_cfg, flags);
> >       if ( IS_ERR(dom0) )
> > -        panic("Error creating domain 0 (rc = %ld)\n", PTR_ERR(dom0));
> > +        panic("Error creating domain %d (rc = %ld)\n", domid, PTR_ERR(dom0));
> >
> >       if ( llc_coloring_enabled && (rc = dom0_set_llc_colors(dom0)) )
> > -        panic("Error initializing LLC coloring for domain 0 (rc = %d)\n", rc);
> > +        panic("Error initializing LLC coloring for domain %pd (rc = %d)\n",
> > +              dom0, rc);
> >
> >       if ( alloc_dom0_vcpu0(dom0) == NULL )
> > -        panic("Error creating domain 0 vcpu0\n");
> > +        panic("Error creating domain %pdv0\n", dom0);
> >
> >       rc = construct_dom0(dom0);
> >       if ( rc )
> > -        panic("Could not set up DOM0 guest OS (rc = %d)\n", rc);
> > +        panic("Could not set up guest OS for domain %pd (rc = %d)\n",
> > +              dom0, rc);
> >
> >       set_xs_domain(dom0);
> >   }
> > diff --git a/xen/arch/arm/setup.c b/xen/arch/arm/setup.c
> > index 10b46d0684..c3959e8d8e 100644
> > --- a/xen/arch/arm/setup.c
> > +++ b/xen/arch/arm/setup.c
> > @@ -418,6 +418,8 @@ void asmlinkage __init start_xen(unsigned long fdt_paddr)
> >
> >       timer_init();
> >
> > +    domid_init();
> > +
> >       init_idle_domain();
> >
> >       rcu_init();
> > diff --git a/xen/arch/x86/setup.c b/xen/arch/x86/setup.c
> > index 2518954124..02f665f520 100644
> > --- a/xen/arch/x86/setup.c
> > +++ b/xen/arch/x86/setup.c
> > @@ -1030,8 +1030,11 @@ static struct domain *__init create_dom0(struct boot_info *bi)
> >       if ( iommu_enabled )
> >           dom0_cfg.flags |= XEN_DOMCTL_CDF_iommu;
> >
> > -    /* Create initial domain.  Not d0 for pvshim. */
> > -    bd->domid = get_initial_domain_id();
> > +    /* Allocate initial domain ID. Not d0 for pvshim. */
> > +    bd->domid = domid_alloc(get_initial_domain_id());
> > +    if ( bd->domid == DOMID_INVALID )
> > +        panic("Error allocating domain ID %d\n", get_initial_domain_id());
> > +
> >       d = domain_create(bd->domid, &dom0_cfg,
> >                         pv_shim ? 0 : CDF_privileged | CDF_hardware);
> >       if ( IS_ERR(d) )
> > @@ -1063,7 +1066,7 @@ static struct domain *__init create_dom0(struct boot_info *bi)
> >
> >           if ( (strlen(acpi_param) == 0) && acpi_disabled )
> >           {
> > -            printk("ACPI is disabled, notifying Domain 0 (acpi=off)\n");
> > +            printk("ACPI is disabled, notifying domain %pd (acpi=off)\n", d);
> >               safe_strcpy(acpi_param, "off");
> >           }
> >
> > @@ -1078,7 +1081,7 @@ static struct domain *__init create_dom0(struct boot_info *bi)
> >
> >       bd->d = d;
> >       if ( construct_dom0(bd) != 0 )
> > -        panic("Could not construct domain 0\n");
> > +        panic("Could not construct domain %pd\n", d);
> >
> >       bd->cmdline = NULL;
> >       xfree(cmdline);
> > @@ -1915,6 +1918,8 @@ void asmlinkage __init noreturn __start_xen(void)
> >       mmio_ro_ranges = rangeset_new(NULL, "r/o mmio ranges",
> >                                     RANGESETF_prettyprint_hex);
> >
> > +    domid_init();
> > +
> >       xsm_multiboot_init(bi);
> >
> >       /*
> > diff --git a/xen/common/device-tree/dom0less-build.c b/xen/common/device-tree/dom0less-build.c
> > index 2c56f13771..9236dbae11 100644
> > --- a/xen/common/device-tree/dom0less-build.c
> > +++ b/xen/common/device-tree/dom0less-build.c
> > @@ -850,15 +850,13 @@ void __init create_domUs(void)
> >           struct xen_domctl_createdomain d_cfg = {0};
> >           unsigned int flags = 0U;
> >           bool has_dtb = false;
> > +        domid_t domid;
> >           uint32_t val;
> >           int rc;
> >
> >           if ( !dt_device_is_compatible(node, "xen,domain") )
> >               continue;
> >
> > -        if ( (max_init_domid + 1) >= DOMID_FIRST_RESERVED )
> > -            panic("No more domain IDs available\n");
> > -
> >           d_cfg.max_evtchn_port = 1023;
> >           d_cfg.max_grant_frames = -1;
> >           d_cfg.max_maptrack_frames = -1;
> > @@ -981,7 +979,11 @@ void __init create_domUs(void)
> >            * very important to use the pre-increment operator to call
> >            * domain_create() with a domid > 0. (domid == 0 is reserved for Dom0)
> >            */
> > -        d = domain_create(++max_init_domid, &d_cfg, flags);
> > +        domid = domid_alloc(++max_init_domid);
> > +        if ( domid == DOMID_INVALID )
> > +            panic("Error allocating ID for domain %s\n", dt_node_name(node));
> > +
> > +        d = domain_create(domid, &d_cfg, flags);
> >           if ( IS_ERR(d) )
> >               panic("Error creating domain %s (rc = %ld)\n",
> >                     dt_node_name(node), PTR_ERR(d));
> > diff --git a/xen/common/domain.c b/xen/common/domain.c
> > index abf1969e60..0ba3cdc47d 100644
> > --- a/xen/common/domain.c
> > +++ b/xen/common/domain.c
> > @@ -66,6 +66,74 @@ DEFINE_RCU_READ_LOCK(domlist_read_lock);
> >   static struct domain *domain_hash[DOMAIN_HASH_SIZE];
> >   struct domain *domain_list;
> >
> > +/* Non-system domain ID allocator. */
> > +static DEFINE_SPINLOCK(domid_lock);
> > +static struct rangeset *domid_rangeset;
> > +static unsigned int domid_last;
> > +
> > +void __init domid_init(void)
> > +{
> > +    domid_rangeset = rangeset_new(NULL, "domid", RANGESETF_prettyprint_hex);
> > +    if ( !domid_rangeset )
> > +        panic("cannot allocate domain ID rangeset\n");
> > +
> > +    rangeset_limit(domid_rangeset, DOMID_FIRST_RESERVED);
> > +}
> > +
> > +/*
> > + * Allocate new non-system domain ID based on the hint.
> > + *
> > + * If hint is outside of valid [0..DOMID_FIRST_RESERVED - 1] range of IDs,
> > + * perform an exhaustive search starting from the end of the used domain ID
> > + * range.
> > + */
> > +domid_t domid_alloc(domid_t domid)
> > +{
> > +    spin_lock(&domid_lock);
> > +
> > +    if ( domid < DOMID_FIRST_RESERVED )
> > +    {
> > +        if ( rangeset_contains_singleton(domid_rangeset, domid) )
> > +            domid = DOMID_INVALID;
> > +    }
> > +    else
> > +    {
> > +        for ( domid = domid_last + 1; domid != domid_last; domid++ )
> > +        {
> > +            if ( domid == DOMID_FIRST_RESERVED )
> > +                domid = 0;
> > +
> > +            if ( !rangeset_contains_singleton(domid_rangeset, domid) )
> > +                break;
> > +        }
> > +
> > +        if ( domid == domid_last )
> > +            domid = DOMID_INVALID;
> > +    }
> > +
> > +    if ( domid != DOMID_INVALID )
> > +    {
> > +        ASSERT(!rangeset_add_singleton(domid_rangeset, domid));
> > +
> > +        if ( domid != domid_last )
> > +            domid_last = domid;
> > +    }
> > +
> > +    spin_unlock(&domid_lock);
> > +
> > +    return domid;
> > +}
> 
> It's mostly a matter of implementation choice, but I am not really fan
> of relying on rangesets, which to me are meant for address ranges or
> something similar but at least large.
> 
> I would rather rely on a bitmap using find_first_zero_bit+set_bit which
> avoids doing a per-domid test, and may be simpler overall. The bitmap
> size for 0x3FF0 domains is almost 4KB, which looks acceptable.
> 
> I don't know what other thinks.

Thanks for taking a look!

TBH, I was initially considering using a bitmap. But then I chose use rangesets
because statically defined bitmap will increase the binary size, which may be
indesirable; and for dynamic allocation, rangeset has all convenience APIs
implemented...

> 
> > +
> > +void domid_free(domid_t domid)
> > +{
> > +    spin_lock(&domid_lock);
> > +
> > +    if ( rangeset_contains_singleton(domid_rangeset, domid) )
> > +        ASSERT(!rangeset_remove_singleton(domid_rangeset, domid));
> > +
> > +    spin_unlock(&domid_lock);
> > +}
> > +
> >   /*
> >    * Insert a domain into the domlist/hash.  This allows the domain to be looked
> >    * up by domid, and therefore to be the subject of hypercalls/etc.
> > @@ -1449,6 +1517,8 @@ void domain_destroy(struct domain *d)
> >
> >       TRACE_TIME(TRC_DOM0_DOM_REM, d->domain_id);
> >
> > +    domid_free(d->domain_id);
> > +
> >       /* Remove from the domlist/hash. */
> >       domlist_remove(d);
> >
> > diff --git a/xen/common/domctl.c b/xen/common/domctl.c
> > index bfe2e1f9f0..2e02139660 100644
> > --- a/xen/common/domctl.c
> > +++ b/xen/common/domctl.c
> > @@ -49,20 +49,6 @@ static int xenctl_bitmap_to_nodemask(nodemask_t *nodemask,
> >                                      MAX_NUMNODES);
> >   }
> >
> > -static inline int is_free_domid(domid_t dom)
> > -{
> > -    struct domain *d;
> > -
> > -    if ( dom >= DOMID_FIRST_RESERVED )
> > -        return 0;
> > -
> > -    if ( (d = rcu_lock_domain_by_id(dom)) == NULL )
> > -        return 1;
> > -
> > -    rcu_unlock_domain(d);
> > -    return 0;
> > -}
> > -
> >   void getdomaininfo(struct domain *d, struct xen_domctl_getdomaininfo *info)
> >   {
> >       struct vcpu *v;
> > @@ -421,34 +407,15 @@ long do_domctl(XEN_GUEST_HANDLE_PARAM(xen_domctl_t) u_domctl)
> >
> >       case XEN_DOMCTL_createdomain:
> >       {
> > -        domid_t        dom;
> > -        static domid_t rover = 0;
> > +        domid_t domid = domid_alloc(op->domain);
> >
> > -        dom = op->domain;
> > -        if ( (dom > 0) && (dom < DOMID_FIRST_RESERVED) )
> > +        if ( domid == DOMID_INVALID )
> >           {
> >               ret = -EEXIST;
> > -            if ( !is_free_domid(dom) )
> > -                break;
> > -        }
> > -        else
> > -        {
> > -            for ( dom = rover + 1; dom != rover; dom++ )
> > -            {
> > -                if ( dom == DOMID_FIRST_RESERVED )
> > -                    dom = 1;
> > -                if ( is_free_domid(dom) )
> > -                    break;
> > -            }
> > -
> > -            ret = -ENOMEM;
> > -            if ( dom == rover )
> > -                break;
> > -
> > -            rover = dom;
> > +            break;
> >           }
> >
> > -        d = domain_create(dom, &op->u.createdomain, false);
> > +        d = domain_create(domid, &op->u.createdomain, false);
> >           if ( IS_ERR(d) )
> >           {
> >               ret = PTR_ERR(d);
> 
> In case the domain creation failure, we need to free the domid,
> otherwise, it would not be used anymore as considered used by the domid
> allocator.

Thanks!

> 
> > diff --git a/xen/include/xen/domain.h b/xen/include/xen/domain.h
> > index e10baf2615..039bb7eeaf 100644
> > --- a/xen/include/xen/domain.h
> > +++ b/xen/include/xen/domain.h
> > @@ -38,6 +38,10 @@ void arch_get_domain_info(const struct domain *d,
> >
> >   domid_t get_initial_domain_id(void);
> >
> > +void domid_init(void);
> > +void domid_free(domid_t domid);
> > +domid_t domid_alloc(domid_t domid);
> > +
> >   /* CDF_* constant. Internal flags for domain creation. */
> >   /* Is this a privileged domain? */
> >   #define CDF_privileged           (1U << 0)
> 
> Teddy
> 
> 
> Teddy Astie | Vates XCP-ng Developer
> 
> XCP-ng & Xen Orchestra - Vates solutions
> 
> web: https://vates.tech
> 
> 
>