[RFC PATCH net-next] net: mana: Force single RX buffer per page under SWIOTLB bounce modes

[RFC PATCH net-next] net: mana: Force single RX buffer per page under SWIOTLB bounce modes

[Dipayaan Roy]

The MANA driver has two distinct DMA paths for RX buffers:

1. Without PP_FLAG_DMA_MAP: The driver maps full pages manually,
   creating page-aligned mappings where the DMA offset is always zero.

2. With PP_FLAG_DMA_MAP: page_pool uses sub-page fragments, where
   multiple RX buffers share a single page. The pool maps the whole
   page once, and subsequent allocations use offsets into that region.

Path 2 is problematic in two scenarios where DMA must go through
SWIOTLB bounce buffers:

- Confidential VMs (AMD SEV-SNP, Intel TDX): guest memory is encrypted
  and the NIC cannot access it directly due to lack of TDISP support.
  All DMA must use SWIOTLB bounce buffers.

- Force-bounce mode (swiotlb=force): all DMA is routed through bounce
  buffers regardless of whether the system is a CVM.

In both cases, sub-page RX buffer fragments allocated via page_pool may
not be compatible with bounce buffering in this mode, leading to failures
in the RX path.

Add a check using is_swiotlb_force_bounce() in
mana_use_single_rxbuf_per_page() to detect when force-bounce is active
for the device and force single RX buffer per page allocation.

Note: This issue likely affects any NIC driver using page_pool with
sub-page fragment allocation under SWIOTLB. A more general fix at
the page_pool level may be desirable. Seeking feedback on the
preferred approach.

Signed-off-by: Dipayaan Roy <dipayanroy@linux.microsoft.com>
---
 drivers/net/ethernet/microsoft/mana/mana_en.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/drivers/net/ethernet/microsoft/mana/mana_en.c b/drivers/net/ethernet/microsoft/mana/mana_en.c
index 2d44eaf932a8..841421baf0de 100644
--- a/drivers/net/ethernet/microsoft/mana/mana_en.c
+++ b/drivers/net/ethernet/microsoft/mana/mana_en.c
@@ -12,6 +12,7 @@
 #include <linux/pci.h>
 #include <linux/export.h>
 #include <linux/skbuff.h>
+#include <linux/swiotlb.h>
 #include <linux/cc_platform.h>
 
 #include <net/checksum.h>
@@ -748,10 +749,15 @@ static void *mana_get_rxbuf_pre(struct mana_rxq *rxq, dma_addr_t *da)
 static bool
 mana_use_single_rxbuf_per_page(struct mana_port_context *apc, u32 mtu)
 {
+	struct gdma_context *gc = apc->ac->gdma_dev->gdma_context;
+
 	/* On confidential VMs with guest memory encryption, all DMA goes
 	 * through SWIOTLB bounce buffers. Sub-page RX fragments may not
 	 * be properly bounce-buffered, so use fullpage buffers.
 	 */
+	if (is_swiotlb_force_bounce(gc->dev))
+		return true;
+
 	if (cc_platform_has(CC_ATTR_GUEST_MEM_ENCRYPT))
 		return true;
 
-- 
2.43.0

Re: [RFC PATCH net-next] net: mana: Force single RX buffer per page under SWIOTLB bounce modes

[Jakub Kicinski]

On Mon, 27 Apr 2026 07:41:11 -0700 Dipayaan Roy wrote:
> In both cases, sub-page RX buffer fragments allocated via page_pool may
> not be compatible with bounce buffering in this mode, leading to failures
> in the RX path.

What does it mean to not be compatible with swiotlb?
Normally that indicates that DMA API is mis-used.