[PATCH v4] x86/pci-dma: add a SWIOTLB_ANY flag to lift the low mem limitation

[Jun Miao <jun.miao@intel.com>]

When high-speed NICs or multi-GPU setups are passed through into confidential
VMs, the SWIOTLB bounce buffer becomes the critical path between private and
shared memory. Restricting it to low memory limits throughput and fails to
scale for larger workloads.

AMD SEV-SNP and Intel TDX guests run in a TEE where the hypervisor is untrusted.
DMA-capable devices require bounce buffers to mediate between encrypted private
memory and unencrypted shared memory. Confining these buffers to low memory (<4GB)
unnecessarily caps their size and degrades performance.

Power SVM already supports this; x86 does not. See commit 8ba2ed1be9
("swiotlb: add a SWIOTLB_ANY flag to lift the low memory restriction").

[ aakarsh: completely trim down/rewrite changelog ]

Tested-by: Aakarsh Jain <aakarsh.jain@oss.qualcomm.com>
Suggested-by: Borislav Petkov <bp@alien8.de>
Acked-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reviewed-by: Aakarsh Jain <aakarsh.jain@oss.qualcomm.com>
Signed-off-by: Jun Miao <jun.miao@intel.com>
---

v1 -> v2:
- Updated commit message and description.
- Add Reviewed and Tested.
V1 Latest Feedback : https://lists.openwall.net/linux-kernel/2026/02/11/483

v2 -> v3:
- We can alloc 4GB with the dynamic swiotlb, rather than 1GB.
  1G is not correct. So change the commit log.

v3 -> v4:
- Not only TDX-specific but all encrypted guests include SEV.
- SEV-SNP guest passed the test with the help of Aakarsh.
  Tested-by: Aakarsh Jain <aakarsh.jain@oss.qualcomm.com>

- Add "Acked-by: Marek Szyprowski"
- Explain the usage case in the commit log following Boris`s suggestion.

---
 arch/x86/kernel/pci-dma.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kernel/pci-dma.c b/arch/x86/kernel/pci-dma.c
index 6267363e0189..73b9320c4a7d 100644
--- a/arch/x86/kernel/pci-dma.c
+++ b/arch/x86/kernel/pci-dma.c
@@ -61,7 +61,7 @@ static void __init pci_swiotlb_detect(void)
 	 */
 	if (cc_platform_has(CC_ATTR_GUEST_MEM_ENCRYPT)) {
 		x86_swiotlb_enable = true;
-		x86_swiotlb_flags |= SWIOTLB_FORCE;
+		x86_swiotlb_flags |= SWIOTLB_ANY | SWIOTLB_FORCE;
 	}
 }
 #else
-- 
2.47.1