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

[Michael Kelley <mhklinux@outlook.com>]

From: Jun Miao <jun.miao@intel.com> Sent: Wednesday, June 24, 2026 6:26 PM
> 
> 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>

I tested this change in Intel TDX and AMD SEV-SNP VMs in the
Azure cloud, which are running on Hyper-V and with a paravisor.
The SEV-SNP VM operates in vTOM mode.

I was able to allocate a 4 GiB swiotlb, whereas prior to this patch
specifying 4 GiB resulted in a memory allocation failure and a
downgrade to 2 GiB. All bounce buffered DMA operations from
the Hyper-V synthetic SCSI and NIC controllers worked correctly,
as did DMA operations from a PCI pass-thru NVMe controller in
the TDX VM.

Reviewed-by: Michael Kelley <mhklinux@outlook.com>
Tested-by: Michael Kelley <mhklinux@outlook.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
>