xen/arm: Don't create useless P2M pool for a hardware domain

On Arm, the hardware domain does not use the P2M pool. Its P2M pages are
allocated directly from the heap, without any limit. However, when the
hardware domain is created using dom0less, we still allocate a P2M pool
for it, either of the default size or the one given in
xen,domain-p2m-mem-mb. This memory is never used and is simply wasted.
The same applies when the pool size is set for the hardware domain using
XEN_DOMCTL_set_paging_mempool_size.

Don't create the pool for a hardware domain. Make the get/set paging
mempool domctls return -EOPNOTSUPP, skip the pool teardown and reject
xen,domain-p2m-mem-mb for a hardware domain in dom0less.

The dom0less check is Arm specific, because on RISC-V the hardware domain
does use the P2M pool.

Signed-off-by: Michal Orzel <michal.orzel@amd.com>
Reviewed-by: Volodymyr Babchuk <volodymyr_babchuk@epam.com>

docs/misc/arm/device-tree/booting.txt

@@ -277,7 +277,9 @@ with the following properties:
     used for the domain P2M pool. This is in-sync with the shadow_memory
     option in xl.cfg. Leaving this field empty in device tree will lead to
     the default size of domain P2M pool, i.e. 1MB per guest vCPU plus 4KB
-    per MB of guest RAM plus 512KB for guest extended regions.
+    per MB of guest RAM plus 512KB for guest extended regions. On Arm, the
+    hardware domain has no dedicated P2M pool, so specifying this property
+    for it makes the domain fail to build.
 
 - max_grant_version
 

xen/arch/arm/mmu/p2m.c

@@ -61,6 +61,10 @@ static void p2m_free_page(struct domain *d, struct page_info *pg)
 /* Return the size of the pool, in bytes. */
 int arch_get_paging_mempool_size(struct domain *d, uint64_t *size)
 {
+    /* No dedicated pool (no limit) for a hardware domain */
+    if ( is_hardware_domain(d) )
+        return -EOPNOTSUPP;
+
     *size = (uint64_t)ACCESS_ONCE(d->arch.paging.p2m_total_pages) << PAGE_SHIFT;
     return 0;
 }
@@ -69,6 +73,8 @@ int p2m_set_allocation(struct domain *d, unsigned long pages, bool can_preempt)
 {
     struct page_info *pg;
 
+    /* No dedicated pool (no limit) for a hardware domain */
+    ASSERT(!is_hardware_domain(d));
     ASSERT(spin_is_locked(&d->arch.paging.lock));
 
     for ( ; ; )
@@ -119,6 +125,10 @@ int arch_set_paging_mempool_size(struct domain *d, uint64_t size)
     unsigned long pages = size >> PAGE_SHIFT;
     int rc;
 
+    /* No dedicated pool (no limit) for a hardware domain */
+    if ( is_hardware_domain(d) )
+        return -EOPNOTSUPP;
+
     if ( (size & ~PAGE_MASK) ||          /* Non page-sized request? */
          pages != (size >> PAGE_SHIFT) ) /* 32-bit overflow? */
         return -EINVAL;
@@ -134,6 +144,10 @@ int p2m_teardown_allocation(struct domain *d)
 {
     int ret = 0;
 
+    /* No dedicated pool - nothing to tear down */
+    if ( is_hardware_domain(d) )
+        return 0;
+
     spin_lock(&d->arch.paging.lock);
     if ( d->arch.paging.p2m_total_pages != 0 )
     {

xen/common/device-tree/dom0less-build.c

@@ -757,6 +757,18 @@ static int __init domain_p2m_set_allocation(struct domain *d, uint64_t mem,
     int rc;
 
     rc = dt_property_read_u32(node, "xen,domain-p2m-mem-mb", &p2m_mem_mb);
+
+    /* No dedicated pool (no limit) for a hardware domain on Arm */
+    if ( IS_ENABLED(CONFIG_ARM) && is_hardware_domain(d) )
+    {
+        if ( !rc )
+            return 0;
+
+        printk(XENLOG_ERR
+               "xen,domain-p2m-mem-mb cannot be specified for a hardware domain\n");
+        return -EINVAL;
+    }
+
     /* If xen,domain-p2m-mem-mb is not specified, use the default value. */
     p2m_pages = rc ?
                 p2m_mem_mb << (20 - PAGE_SHIFT) :