KVM: arm64: Allow attaching of non-coalescable pages to a hyp pool

All the contiguous pages used to initialize a 'struct hyp_pool' are
considered coalescable, which means that the hyp page allocator will
actively try to merge them with their buddies on the hyp_put_page() path.
However, using hyp_put_page() on a page that is not part of the inital
memory range given to a hyp_pool() is currently unsupported.

In order to allow dynamically extending hyp pools at run-time, add a
check to __hyp_attach_page() to allow inserting 'external' pages into
the free-list of order 0. This will be necessary to allow lazy donation
of pages from the host to the hypervisor when allocating guest stage-2
page-table pages at EL2.

Tested-by: Vincent Donnefort <vdonnefort@google.com>
Signed-off-by: Quentin Perret <qperret@google.com>
Signed-off-by: Will Deacon <will@kernel.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20221110190259.26861-3-will@kernel.org

arch/arm64/kvm/hyp/nvhe/page_alloc.c

@@ -93,11 +93,16 @@ static inline struct hyp_page *node_to_page(struct list_head *node)
 static void __hyp_attach_page(struct hyp_pool *pool,
 			      struct hyp_page *p)
 {
+	phys_addr_t phys = hyp_page_to_phys(p);
 	unsigned short order = p->order;
 	struct hyp_page *buddy;
 
 	memset(hyp_page_to_virt(p), 0, PAGE_SIZE << p->order);
 
+	/* Skip coalescing for 'external' pages being freed into the pool. */
+	if (phys < pool->range_start || phys >= pool->range_end)
+		goto insert;
+
 	/*
 	 * Only the first struct hyp_page of a high-order page (otherwise known
 	 * as the 'head') should have p->order set. The non-head pages should
@@ -116,6 +121,7 @@ static void __hyp_attach_page(struct hyp_pool *pool,
 		p = min(p, buddy);
 	}
 
+insert:
 	/* Mark the new head, and insert it */
 	p->order = order;
 	page_add_to_list(p, &pool->free_area[order]);