KVM: arm64: Remove size-order align in the nVHE hyp private VA range

commit f922c13e778d ("KVM: arm64: Introduce
pkvm_alloc_private_va_range()") and commit 92abe0f81e13 ("KVM: arm64:
Introduce hyp_alloc_private_va_range()") added an alignment for the
start address of any allocation into the nVHE hypervisor private VA
range.

This alignment (order of the size of the allocation) intends to enable
efficient stack verification (if the PAGE_SHIFT bit is zero, the stack
pointer is on the guard page and a stack overflow occurred).

But this is only necessary for stack allocation and can waste a lot of
VA space. So instead make stack-specific functions, handling the guard
page requirements, while other users (e.g.  fixmap) will only get page
alignment.

Reviewed-by: Kalesh Singh <kaleshsingh@google.com>
Signed-off-by: Vincent Donnefort <vdonnefort@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20230811112037.1147863-1-vdonnefort@google.com

1 files changed, 68 insertions, 17 deletions

diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
index 137b775d238b..499555e04732 100644
--- a/arch/arm64/kvm/mmu.c
+++ b/arch/arm64/kvm/mmu.c
@@ -592,6 +592,25 @@ int create_hyp_mappings(void *from, void *to, enum kvm_pgtable_prot prot)
 	return 0;
 }
 
+static int __hyp_alloc_private_va_range(unsigned long base)
+{
+	lockdep_assert_held(&kvm_hyp_pgd_mutex);
+
+	if (!PAGE_ALIGNED(base))
+		return -EINVAL;
+
+	/*
+	 * Verify that BIT(VA_BITS - 1) hasn't been flipped by
+	 * allocating the new area, as it would indicate we've
+	 * overflowed the idmap/IO address range.
+	 */
+	if ((base ^ io_map_base) & BIT(VA_BITS - 1))
+		return -ENOMEM;
+
+	io_map_base = base;
+
+	return 0;
+}
 
 /**
  * hyp_alloc_private_va_range - Allocates a private VA range.
@@ -612,26 +631,16 @@ int hyp_alloc_private_va_range(size_t size, unsigned long *haddr)
 
 	/*
 	 * This assumes that we have enough space below the idmap
-	 * page to allocate our VAs. If not, the check below will
-	 * kick. A potential alternative would be to detect that
-	 * overflow and switch to an allocation above the idmap.
+	 * page to allocate our VAs. If not, the check in
+	 * __hyp_alloc_private_va_range() will kick. A potential
+	 * alternative would be to detect that overflow and switch
+	 * to an allocation above the idmap.
 	 *
 	 * The allocated size is always a multiple of PAGE_SIZE.
 	 */
-	base = io_map_base - PAGE_ALIGN(size);
-
-	/* Align the allocation based on the order of its size */
-	base = ALIGN_DOWN(base, PAGE_SIZE << get_order(size));
-
-	/*
-	 * Verify that BIT(VA_BITS - 1) hasn't been flipped by
-	 * allocating the new area, as it would indicate we've
-	 * overflowed the idmap/IO address range.
-	 */
-	if ((base ^ io_map_base) & BIT(VA_BITS - 1))
-		ret = -ENOMEM;
-	else
-		*haddr = io_map_base = base;
+	size = PAGE_ALIGN(size);
+	base = io_map_base - size;
+	ret = __hyp_alloc_private_va_range(base);
 
 	mutex_unlock(&kvm_hyp_pgd_mutex);
 
@@ -668,6 +677,48 @@ static int __create_hyp_private_mapping(phys_addr_t phys_addr, size_t size,
 	return ret;
 }
 
+int create_hyp_stack(phys_addr_t phys_addr, unsigned long *haddr)
+{
+	unsigned long base;
+	size_t size;
+	int ret;
+
+	mutex_lock(&kvm_hyp_pgd_mutex);
+	/*
+	 * Efficient stack verification using the PAGE_SHIFT bit implies
+	 * an alignment of our allocation on the order of the size.
+	 */
+	size = PAGE_SIZE * 2;
+	base = ALIGN_DOWN(io_map_base - size, size);
+
+	ret = __hyp_alloc_private_va_range(base);
+
+	mutex_unlock(&kvm_hyp_pgd_mutex);
+
+	if (ret) {
+		kvm_err("Cannot allocate hyp stack guard page\n");
+		return ret;
+	}
+
+	/*
+	 * Since the stack grows downwards, map the stack to the page
+	 * at the higher address and leave the lower guard page
+	 * unbacked.
+	 *
+	 * Any valid stack address now has the PAGE_SHIFT bit as 1
+	 * and addresses corresponding to the guard page have the
+	 * PAGE_SHIFT bit as 0 - this is used for overflow detection.
+	 */
+	ret = __create_hyp_mappings(base + PAGE_SIZE, PAGE_SIZE, phys_addr,
+				    PAGE_HYP);
+	if (ret)
+		kvm_err("Cannot map hyp stack\n");
+
+	*haddr = base + size;
+
+	return ret;
+}
+
 /**
  * create_hyp_io_mappings - Map IO into both kernel and HYP
  * @phys_addr:	The physical start address which gets mapped