From cff43e997f79a95dc44e02debaeafe5f127f40bb Mon Sep 17 00:00:00 2001
From: Juergen Gross <jgross@xxxxxxxx>
Date: Thu, 30 Jan 2025 09:56:57 +0100
Subject: [PATCH] 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 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 the early_boot_irqs_disabled
flag.

Fixes: 9f40ec84a797 ("xen/swiotlb: add alignment check for dma buffers")
Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
---
Note that the "Fixes:" tag is not really correct, as that patch didn't
introduce the problem, but rather made it visible. OTOH it is the best
indicator we have to identify kernel versions this patch should be
backported to.
---
 arch/x86/xen/mmu_pv.c | 44 ++++++++++++++++++++++++++++++++++++-------
 1 file changed, 37 insertions(+), 7 deletions(-)

diff --git a/arch/x86/xen/mmu_pv.c b/arch/x86/xen/mmu_pv.c
index 55a4996d0c04..62aec29b8174 100644
--- a/arch/x86/xen/mmu_pv.c
+++ b/arch/x86/xen/mmu_pv.c
@@ -2200,8 +2200,10 @@ void __init xen_init_mmu_ops(void)
 }
 
 /* Protected by xen_reservation_lock. */
-#define MAX_CONTIG_ORDER 9 /* 2MB */
-static unsigned long discontig_frames[1<<MAX_CONTIG_ORDER];
+#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];
+static unsigned long *discontig_frames = discontig_frames_early;
 
 #define VOID_PTE (mfn_pte(0, __pgprot(0)))
 static void xen_zap_pfn_range(unsigned long vaddr, unsigned int order,
@@ -2319,18 +2321,44 @@ 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 *new_array, *old_array;
 	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 (early_boot_irqs_disabled)
+			return -ENOMEM;
+
+		new_array = vmalloc(sizeof(unsigned long) * (1UL << order));
+
+		if (!new_array)
+			return -ENOMEM;
+
+		spin_lock_irqsave(&xen_reservation_lock, flags);
+
+		if (order > discontig_frames_order) {
+			if (discontig_frames == discontig_frames_early)
+				old_array = NULL;
+			else
+				old_array = discontig_frames;
+			discontig_frames = new_array;
+			discontig_frames_order = order;
+		} else
+			old_array = new_array;
+
+		spin_unlock_irqrestore(&xen_reservation_lock, flags);
+
+		vfree(old_array);
+	}
 
 	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);
 
@@ -2354,12 +2382,12 @@ 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);
@@ -2367,6 +2395,8 @@ void xen_destroy_contiguous_region(phys_addr_t pstart, unsigned int 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);
 
-- 
2.43.0