x86/xen: allow larger contiguous memory regions in PV guests

Today a PV guest (including dom0) can create 2MB contiguous memory
regions for DMA buffers at max. This has led to problems at least
with the megaraid_sas driver, which wants to allocate a 2.3MB DMA
buffer.

The limiting factor is the frame array used to do the hypercall for
making the memory contiguous, which has 512 entries and is just a
static array in mmu_pv.c.

In order to not waste memory for non-PV guests, put the initial
frame array into .init.data section and dynamically allocate an array
from the .init_after_bootmem hook of PV guests.

In case a contiguous memory area larger than the initially supported
2MB is requested, allocate a larger buffer for the frame list. Note
that such an allocation is tried only after memory management has been
initialized properly, which is tested via a flag being set in the
.init_after_bootmem hook.

Fixes: 9f40ec8 ("xen/swiotlb: add alignment check for dma buffers")
Signed-off-by: Juergen Gross <[email protected]>
Tested-by: Alan Robinson <[email protected]>
Reviewed-by: Jan Beulich <[email protected]>
Signed-off-by: Juergen Gross <[email protected]>

Author: jgross1

arch/x86/xen/mmu_pv.c

@@ -111,6 +111,51 @@ static pud_t level3_user_vsyscall[PTRS_PER_PUD] __page_aligned_bss;
  */
 static DEFINE_SPINLOCK(xen_reservation_lock);
 
+/* Protected by xen_reservation_lock. */
+#define MIN_CONTIG_ORDER 9 /* 2MB */
+static unsigned int discontig_frames_order = MIN_CONTIG_ORDER;
+static unsigned long discontig_frames_early[1UL << MIN_CONTIG_ORDER] __initdata;
+static unsigned long *discontig_frames __refdata = discontig_frames_early;
+static bool discontig_frames_dyn;
+
+static int alloc_discontig_frames(unsigned int order)
+{
+	unsigned long *new_array, *old_array;
+	unsigned int old_order;
+	unsigned long flags;
+
+	BUG_ON(order < MIN_CONTIG_ORDER);
+	BUILD_BUG_ON(sizeof(discontig_frames_early) != PAGE_SIZE);
+
+	new_array = (unsigned long *)__get_free_pages(GFP_KERNEL,
+						      order - MIN_CONTIG_ORDER);
+	if (!new_array)
+		return -ENOMEM;
+
+	spin_lock_irqsave(&xen_reservation_lock, flags);
+
+	old_order = discontig_frames_order;
+
+	if (order > discontig_frames_order || !discontig_frames_dyn) {
+		if (!discontig_frames_dyn)
+			old_array = NULL;
+		else
+			old_array = discontig_frames;
+
+		discontig_frames = new_array;
+		discontig_frames_order = order;
+		discontig_frames_dyn = true;
+	} else {
+		old_array = new_array;
+	}
+
+	spin_unlock_irqrestore(&xen_reservation_lock, flags);
+
+	free_pages((unsigned long)old_array, old_order - MIN_CONTIG_ORDER);
+
+	return 0;
+}
+
 /*
  * Note about cr3 (pagetable base) values:
  *
@@ -814,6 +859,9 @@ static void __init xen_after_bootmem(void)
 	SetPagePinned(virt_to_page(level3_user_vsyscall));
 #endif
 	xen_pgd_walk(&init_mm, xen_mark_pinned, FIXADDR_TOP);
+
+	if (alloc_discontig_frames(MIN_CONTIG_ORDER))
+		BUG();
 }
 
 static void xen_unpin_page(struct mm_struct *mm, struct page *page,
@@ -2203,10 +2251,6 @@ void __init xen_init_mmu_ops(void)
 	memset(dummy_mapping, 0xff, PAGE_SIZE);
 }
 
-/* Protected by xen_reservation_lock. */
-#define MAX_CONTIG_ORDER 9 /* 2MB */
-static unsigned long discontig_frames[1<<MAX_CONTIG_ORDER];
-
 #define VOID_PTE (mfn_pte(0, __pgprot(0)))
 static void xen_zap_pfn_range(unsigned long vaddr, unsigned int order,
 				unsigned long *in_frames,
@@ -2323,18 +2367,25 @@ int xen_create_contiguous_region(phys_addr_t pstart, unsigned int order,
 				 unsigned int address_bits,
 				 dma_addr_t *dma_handle)
 {
-	unsigned long *in_frames = discontig_frames, out_frame;
+	unsigned long *in_frames, out_frame;
 	unsigned long  flags;
 	int            success;
 	unsigned long vstart = (unsigned long)phys_to_virt(pstart);
 
-	if (unlikely(order > MAX_CONTIG_ORDER))
-		return -ENOMEM;
+	if (unlikely(order > discontig_frames_order)) {
+		if (!discontig_frames_dyn)
+			return -ENOMEM;
+
+		if (alloc_discontig_frames(order))
+			return -ENOMEM;
+	}
 
 	memset((void *) vstart, 0, PAGE_SIZE << order);
 
 	spin_lock_irqsave(&xen_reservation_lock, flags);
 
+	in_frames = discontig_frames;
+
 	/* 1. Zap current PTEs, remembering MFNs. */
 	xen_zap_pfn_range(vstart, order, in_frames, NULL);
 
@@ -2358,19 +2409,21 @@ int xen_create_contiguous_region(phys_addr_t pstart, unsigned int order,
 
 void xen_destroy_contiguous_region(phys_addr_t pstart, unsigned int order)
 {
-	unsigned long *out_frames = discontig_frames, in_frame;
+	unsigned long *out_frames, in_frame;
 	unsigned long  flags;
 	int success;
 	unsigned long vstart;
 
-	if (unlikely(order > MAX_CONTIG_ORDER))
+	if (unlikely(order > discontig_frames_order))
 		return;
 
 	vstart = (unsigned long)phys_to_virt(pstart);
 	memset((void *) vstart, 0, PAGE_SIZE << order);
 
 	spin_lock_irqsave(&xen_reservation_lock, flags);
 
+	out_frames = discontig_frames;
+
 	/* 1. Find start MFN of contiguous extent. */
 	in_frame = virt_to_mfn((void *)vstart);