Re: [PATCH v13 1/3] xen/domain: unify domain ID allocation

[dmkhn@proton.me]

On Tue, Aug 05, 2025 at 03:38:37PM +0200, Roger Pau Monné wrote:
> On Wed, Jul 30, 2025 at 05:40:54PM +0000, dmkhn@proton.me wrote:
> > From: Denis Mukhin <dmukhin@ford.com>
> >
> > Currently, there are two different domain ID allocation implementations:
> >
> >   1) Sequential IDs allocation in dom0less Arm code based on max_init_domid;
> >
> >   2) Sequential IDs allocation in XEN_DOMCTL_createdomain; does not use
> >      max_init_domid (both Arm and x86).
> >
> > The domain ID allocation covers dom0 or late hwdom, predefined domains,
> > post-boot domains, excluding Xen system domains (domid >=
> > DOMID_FIRST_RESERVED).
> >
> > It makes sense to have a common helper code for such task across architectures
> > (Arm and x86) and between dom0less / toolstack domU allocation.
> >
> > Note, fixing dependency on max_init_domid is out of scope of this patch.
> >
> > Wrap the domain ID allocation as an arch-independent function domid_alloc() in
> > new common/domid.c based on the bitmap.
> >
> > Allocation algorithm:
> > - If an explicit domain ID is provided, verify its availability and use it if
> >   ID is not used;
> > - If DOMID_INVALID is provided, search the range [1..DOMID_FIRST_RESERVED-1],
> >   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.
> >
> > Remove is_free_domid() helper as it is not needed now.
> >
> > No functional change intended.
> >
> > Signed-off-by: Denis Mukhin <dmukhin@ford.com>
> > Reviewed-by: Alejandro Vallejo <alejandro.garciavallejo@amd.com>
> > ---
> > Changes since v12:
> > - updated comment for domid_alloc() and commit message
> > - added Alejandro's R-b
> > ---
> >  xen/arch/arm/domain_build.c             |  7 +-
> >  xen/arch/x86/setup.c                    |  7 +-
> >  xen/common/Makefile                     |  1 +
> >  xen/common/device-tree/dom0less-build.c | 15 ++--
> >  xen/common/domain.c                     |  2 +
> >  xen/common/domctl.c                     | 42 ++---------
> >  xen/common/domid.c                      | 94 +++++++++++++++++++++++++
> >  xen/include/xen/domain.h                |  3 +
> >  8 files changed, 124 insertions(+), 47 deletions(-)
> >  create mode 100644 xen/common/domid.c
> >
> > diff --git a/xen/arch/arm/domain_build.c b/xen/arch/arm/domain_build.c
> > index 463ae4474d30..789f2b9d3ce7 100644
> > --- a/xen/arch/arm/domain_build.c
> > +++ b/xen/arch/arm/domain_build.c
> > @@ -2050,6 +2050,7 @@ 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 */
> > @@ -2074,7 +2075,11 @@ 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));
> >
> > diff --git a/xen/arch/x86/setup.c b/xen/arch/x86/setup.c
> > index 1543dd251cc6..2ff7c28c277b 100644
> > --- a/xen/arch/x86/setup.c
> > +++ b/xen/arch/x86/setup.c
> > @@ -1047,8 +1047,11 @@ 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());
> 
> Nit: in other error messages in the same function we handle the domid
> as an unsigned integer, so %u probably wants using here.  Unless you
> have an explicit intention to print IDs >= DOMID_FIRST_RESERVED as
> negative integers?

No negative integers, that should be %u
Thanks!

> 
> > +
> >      d = domain_create(bd->domid, &dom0_cfg,
> >                        pv_shim ? 0 : CDF_privileged | CDF_hardware);
> >      if ( IS_ERR(d) )
> > diff --git a/xen/common/Makefile b/xen/common/Makefile
> > index c316957fcb36..0c7d0f5d46e1 100644
> > --- a/xen/common/Makefile
> > +++ b/xen/common/Makefile
> > @@ -11,6 +11,7 @@ obj-$(filter-out $(CONFIG_X86),$(CONFIG_ACPI)) += device.o
> >  obj-$(CONFIG_DEVICE_TREE_PARSE) += device-tree/
> >  obj-$(CONFIG_IOREQ_SERVER) += dm.o
> >  obj-y += domain.o
> > +obj-y += domid.o
> >  obj-y += event_2l.o
> >  obj-y += event_channel.o
> >  obj-$(CONFIG_EVTCHN_FIFO) += event_fifo.o
> > diff --git a/xen/common/device-tree/dom0less-build.c b/xen/common/device-tree/dom0less-build.c
> > index 6bb038111de9..f4b6b515d2d2 100644
> > --- a/xen/common/device-tree/dom0less-build.c
> > +++ b/xen/common/device-tree/dom0less-build.c
> > @@ -833,6 +833,7 @@ void __init create_domUs(void)
> >      {
> >          struct kernel_info ki = KERNEL_INFO_INIT;
> >          int rc = parse_dom0less_node(node, &ki.bd);
> > +        domid_t domid;
> >
> >          if ( rc == -ENOENT )
> >              continue;
> > @@ -842,13 +843,13 @@ void __init create_domUs(void)
> >          if ( (max_init_domid + 1) >= DOMID_FIRST_RESERVED )
> >              panic("No more domain IDs available\n");
> >
> > -        /*
> > -         * The variable max_init_domid is initialized with zero, so here it's
> > -         * very important to use the pre-increment operator to call
> > -         * domain_create() with a domid > 0. (domid == 0 is reserved for Dom0)
> > -         */
> > -        ki.bd.d = domain_create(++max_init_domid,
> > -                                &ki.bd.create_cfg, ki.bd.create_flags);
> > +        domid = domid_alloc(DOMID_INVALID);
> > +        if ( domid == DOMID_INVALID )
> > +            panic("Error allocating ID for domain %s\n", dt_node_name(node));
> > +
> > +        max_init_domid = max(max_init_domid, domid);
> > +
> > +        ki.bd.d = domain_create(domid, &ki.bd.create_cfg, ki.bd.create_flags);
> >          if ( IS_ERR(ki.bd.d) )
> >              panic("Error creating domain %s (rc = %ld)\n",
> >                    dt_node_name(node), PTR_ERR(ki.bd.d));
> > diff --git a/xen/common/domain.c b/xen/common/domain.c
> > index 5241a1629eeb..12fbab01cd8e 100644
> > --- a/xen/common/domain.c
> > +++ b/xen/common/domain.c
> > @@ -1473,6 +1473,8 @@ void domain_destroy(struct domain *d)
> >      /* Remove from the domlist/hash. */
> >      domlist_remove(d);
> >
> > +    domid_free(d->domain_id);
> 
> The domlist removal above still allows current users to continue
> "operating" on the domain until the next RCU.  Should for safety the
> freeing of the domid be deferred to _domain_destroy(), which is
> executed in RCU context, and thus ensures there are no current users
> of the removed domain?
> 
> I cannot think of a specific scenario where this could be dangerous
> right now, but deferring to RCU context together with the final
> cleanup seems safer overall.

I agree, _domain_destroy() is the place for domid_free().

> 
> > +
> >      /* Schedule RCU asynchronous completion of domain destroy. */
> >      call_rcu(&d->rcu, complete_domain_destroy);
> >  }
> > diff --git a/xen/common/domctl.c b/xen/common/domctl.c
> > index f2a7caaf853c..5509998aa139 100644
> > --- a/xen/common/domctl.c
> > +++ b/xen/common/domctl.c
> > @@ -51,20 +51,6 @@ 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;
> > @@ -423,36 +409,18 @@ 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 )
> 
> This is a change in behavior AFAICT, as you now allow
> XEN_DOMCTL_createdomain to possibly create a domain with domid 0 (if
> it's available).  Currently op->domain == 0 is handled as op->domain
> == DOMID_FIRST_RESERVED. You either need to adjust the code here, so
> that you do:
> 
> domid_t domid = domid_alloc(op->domain ?: DOMID_FIRST_RESERVED);
> 
> Or domid_alloc() needs to be adjusted to handle an input domid == 0 as
> it handles DOMID_FIRST_RESERVED.

Thanks!
I will adjust code in do_domctl().

> 
> >          {
> >              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) )
> >          {
> > +            domid_free(domid);
> >              ret = PTR_ERR(d);
> >              d = NULL;
> >              break;
> > diff --git a/xen/common/domid.c b/xen/common/domid.c
> > new file mode 100644
> > index 000000000000..e727dcaf0793
> > --- /dev/null
> > +++ b/xen/common/domid.c
> > @@ -0,0 +1,94 @@
> > +/* SPDX-License-Identifier: GPL-2.0-only */
> > +/*
> > + * Domain ID allocator.
> > + *
> > + * Covers dom0 or late hwdom, predefined domains, post-boot domains.
> > + * Excludes Xen 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 = find_next_zero_bit(domid_bitmap,
> > +                                   DOMID_FIRST_RESERVED,
> > +                                   domid_last + 1);
> > +        if ( domid == DOMID_FIRST_RESERVED )
> 
> Nit: you could further gate this second search to domid_last != 0, as
> otherwise the first search has already scanned the whole bitmap.

Ack.

> 
> > +            domid = find_next_zero_bit(domid_bitmap,
> > +                                       DOMID_FIRST_RESERVED,
> > +                                       1);
> 
> Nit: you could possibly limit this second search to (domid_last + 1)
> size, as you have already searched from [domid_last + 1,
> DOMID_FIRST_RESERVED], and the bitmap couldn't have changed as the
> lock is being held.

Ack.

> 
> Thanks, Roger.