[PATCH 1/2] Swiotlb: Add swiotlb_alloc_from_low_pages switch

[Tianyu Lan <ltykernel@gmail.com>]

From: Tianyu Lan <Tianyu.Lan@microsoft.com>

Hyper-V Isolation VM and AMD SEV VM uses swiotlb bounce buffer to
share memory with hypervisor. Current swiotlb bounce buffer is only
allocated from 0 to ARCH_LOW_ADDRESS_LIMIT which is default to
0xffffffffUL. Isolation VM and AMD SEV VM needs 1G bounce buffer at most.
This will fail when there is not enough contiguous memory from 0 to 4G
address space and devices also may use memory above 4G address space as
DMA memory. Expose swiotlb_alloc_from_low_pages and platform mey set it
to false when it's not necessary to limit bounce buffer from 0 to 4G memory.

Signed-off-by: Tianyu Lan <Tianyu.Lan@microsoft.com>
---
 include/linux/swiotlb.h |  1 +
 kernel/dma/swiotlb.c    | 13 +++++++++++--
 2 files changed, 12 insertions(+), 2 deletions(-)

diff --git a/include/linux/swiotlb.h b/include/linux/swiotlb.h
index f6c3638255d5..55c178e8eee0 100644
--- a/include/linux/swiotlb.h
+++ b/include/linux/swiotlb.h
@@ -191,5 +191,6 @@ static inline bool is_swiotlb_for_alloc(struct device *dev)
 #endif /* CONFIG_DMA_RESTRICTED_POOL */
 
 extern phys_addr_t swiotlb_unencrypted_base;
+extern bool swiotlb_alloc_from_low_pages;
 
 #endif /* __LINUX_SWIOTLB_H */
diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
index f1e7ea160b43..159fef80f3db 100644
--- a/kernel/dma/swiotlb.c
+++ b/kernel/dma/swiotlb.c
@@ -73,6 +73,12 @@ enum swiotlb_force swiotlb_force;
 
 struct io_tlb_mem io_tlb_default_mem;
 
+/*
+ * Get IO TLB memory from the low pages if swiotlb_alloc_from_low_pages
+ * is set.
+ */
+bool swiotlb_alloc_from_low_pages = true;
+
 phys_addr_t swiotlb_unencrypted_base;
 
 /*
@@ -284,8 +290,11 @@ swiotlb_init(int verbose)
 	if (swiotlb_force == SWIOTLB_NO_FORCE)
 		return;
 
-	/* Get IO TLB memory from the low pages */
-	tlb = memblock_alloc_low(bytes, PAGE_SIZE);
+	if (swiotlb_alloc_from_low_pages)
+		tlb = memblock_alloc_low(bytes, PAGE_SIZE);
+	else
+		tlb = memblock_alloc(bytes, PAGE_SIZE);
+
 	if (!tlb)
 		goto fail;
 	if (swiotlb_init_with_tbl(tlb, default_nslabs, verbose))
-- 
2.25.1