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Subject[PATCH 1/2] Swiotlb: Add swiotlb_alloc_from_low_pages switch
Date
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
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