[RFC PATCH v7 13/13] swiotlb: Make rounded shared pool capacity allocatable

["Aneesh Kumar K.V (Arm)" <aneesh.kumar@kernel.org>]

CoCo shared memory may need to be allocated and transitioned in units larger
than the requested object. Before this change, users handled the resulting
capacity as follows:

  User                         Rounded capacity reused
  dma-buf system heap          no
  DMA-direct                   no
  regular GIC tables           no
  small GIC ITTs               yes, through a gen_pool
  early SWIOTLB pool           no
  late SWIOTLB pool            yes
  persistent dynamic SWIOTLB   no
  transient dynamic SWIOTLB    no, one mapping only
  atomic DMA pools             yes, through a gen_pool
  restricted SWIOTLB pool      no additional padding

Improve the early and persistent dynamic SWIOTLB pools. They already own
and transition backing rounded to the shared granule size, and SWIOTLB
is itself a suballocator. Advertise the rounded extent as slots, size
the slot metadata to match. This makes the extra capacity available
without reserving more backing memory.

Keep transient dynamic pools unchanged. A transient pool belongs to one
DMA mapping and is destroyed when that mapping is unmapped, so its spare
backing cannot satisfy a later request without changing the lifetime
model.

Do not attempt the same optimization for dma-buf, DMA-direct or regular
GIC objects. Those allocations have independent caller-visible sizes and
lifetimes. Reusing their padding requires a shared-granule suballocator
with reference counting, per-object mappings and accounting.

Note:
For the current 64 KiB CCA shared granule size, SWIOTLB pool sizes are
already multiples of the 256 KiB IO_TLB segment size. Consequently, the
rounding does not change any runtime values on current CCA systems. It
instead makes the code express the intended invariant that pool metadata
describes the complete shared-granule-aligned backing allocation.

Signed-off-by: Aneesh Kumar K.V (Arm) <aneesh.kumar@kernel.org>
---
 kernel/dma/swiotlb.c | 30 ++++++++++++++++++++++++------
 1 file changed, 24 insertions(+), 6 deletions(-)

diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
index cb67105b8812..9577a8807b07 100644
--- a/kernel/dma/swiotlb.c
+++ b/kernel/dma/swiotlb.c
@@ -330,6 +330,14 @@ static inline unsigned long nr_slots(u64 val)
 	return DIV_ROUND_UP(val, IO_TLB_SIZE);
 }
 
+static unsigned long swiotlb_align_nslabs(unsigned long nslabs)
+{
+	unsigned long granule_nslabs;
+
+	granule_nslabs = cc_shared_granule_size() >> IO_TLB_SHIFT;
+	return ALIGN(nslabs, granule_nslabs);
+}
+
 static void swiotlb_mark_pool_used(struct io_tlb_pool *pool)
 {
 	unsigned long i;
@@ -435,11 +443,12 @@ static void add_mem_pool(struct io_tlb_mem *mem, struct io_tlb_pool *pool)
 }
 
 static void __init *swiotlb_memblock_alloc(unsigned long nslabs,
-		unsigned int flags,
+		unsigned long *alloc_nslabs, unsigned int flags,
 		int (*remap)(void *tlb, unsigned long nslabs))
 {
+	unsigned long aligned_nslabs = swiotlb_align_nslabs(nslabs);
+	size_t bytes = aligned_nslabs << IO_TLB_SHIFT;
 	void *tlb;
-	size_t bytes = ALIGN(nslabs << IO_TLB_SHIFT, cc_shared_granule_size());
 
 	/*
 	 * By default allocate the bounce buffer memory from low memory, but
@@ -457,12 +466,13 @@ static void __init *swiotlb_memblock_alloc(unsigned long nslabs,
 		return NULL;
 	}
 
-	if (remap && remap(tlb, nslabs) < 0) {
+	if (remap && remap(tlb, aligned_nslabs) < 0) {
 		memblock_free(tlb, bytes);
 		pr_warn("%s: Failed to remap %zu bytes\n", __func__, bytes);
 		return NULL;
 	}
 
+	*alloc_nslabs = aligned_nslabs;
 	return tlb;
 }
 
@@ -475,6 +485,7 @@ void __init swiotlb_init_remap(bool addressing_limit, unsigned int flags,
 {
 	struct io_tlb_pool *mem = &io_tlb_default_mem.defpool;
 	unsigned long nslabs;
+	unsigned long alloc_nslabs;
 	unsigned int nareas;
 	size_t alloc_size;
 	void *tlb;
@@ -499,13 +510,14 @@ void __init swiotlb_init_remap(bool addressing_limit, unsigned int flags,
 		swiotlb_adjust_nareas(num_possible_cpus());
 
 	nslabs = default_nslabs;
-	nareas = limit_nareas(default_nareas, nslabs);
-	while ((tlb = swiotlb_memblock_alloc(nslabs, flags, remap)) == NULL) {
+	while ((tlb = swiotlb_memblock_alloc(nslabs, &alloc_nslabs, flags,
+					     remap)) == NULL) {
 		if (nslabs <= IO_TLB_MIN_SLABS)
 			return;
 		nslabs = ALIGN(nslabs >> 1, IO_TLB_SEGSIZE);
-		nareas = limit_nareas(nareas, nslabs);
 	}
+	nslabs = alloc_nslabs;
+	nareas = limit_nareas(default_nareas, nslabs);
 
 	if (default_nslabs != nslabs) {
 		pr_info("SWIOTLB bounce buffer size adjusted %lu -> %lu slabs",
@@ -871,6 +883,12 @@ static struct io_tlb_pool *swiotlb_alloc_pool(struct device *dev,
 		tlb_size = nslabs << IO_TLB_SHIFT;
 	}
 
+	/* Transient pools are tied to one mapping and cannot reuse padding. */
+	if (mem->cc_shared && !dev) {
+		nslabs = swiotlb_align_nslabs(nslabs);
+		tlb_size = nslabs << IO_TLB_SHIFT;
+	}
+
 	slot_order = get_order(array_size(sizeof(*pool->slots), nslabs));
 	pool->slots = (struct io_tlb_slot *)
 		__get_free_pages(gfp, slot_order);
-- 
2.43.0