RE: [PATCH v3] x86/pci-dma: add a SWIOTLB_ANY flag to lift the low mem limitation

[Miao, Jun]

>On Wed, Jun 24, 2026 at 01:53:39AM +0000, Miao, Jun wrote:
>> Good suggestion and thank you for testing the ADM SEV-SNP.
>
>I don't think anyone tested it on SNP yet.

I only know that SEV-SNP is a more advanced, third-generation feature.
Are we using "SEV" here as a shorthand for these?

>
>> Confidential Virtual Machines (CVMs) include Intel TDX and AMD SEV,
>> SEV-ES, SEV-SNP want to allocate a swiotlb buffer that is not
>> restricted to low memory for
>
>Why do they want that? The "why" is what I am missing in all this text.

This was an oversight on my part. I failed to explain the usage context:

When using confidential VMs, users want to pass through both the high-speed network interface card (NIC) 
or an 8-GPUs setup into the CVMs. During data transfer, the SWIOTLB bounce buffer becomes a 
critical "hot path" acting as an intermediary for convertor between private and shared memory. 
consequently, the capacity requirement increases—otherwise, network or data transfer performance would be adversely affected.

>
>> the trusted hypervisor scheme.
>
>What is the "trusted hypervisor scheme"?

What I mean to convey is that in TEE environments based on AMD SEV or Intel TDX, 
the core issue is the lack of trust in the hypervisor's VMM.

>
>> Adding a new flag will be used in swiotlb_memblock_alloc() to allow
>> allocation of SWIOTLB memory beyond low memory restriction.
>
>That you don't need to explain - that's visible from the diff.

Ok drop it!
What about the commit description like this in V4 ?

When using confidential VMs, users want to pass through both the high-speed network interface card (NIC) 
or an 8-GPUs setup into the CVMs. During data transfer, the SWIOTLB bounce buffer becomes a 
critical "hot path" acting as an intermediary for convertor between private and shared memory. 
consequently, the capacity requirement increases—otherwise, network or data transfer performance would be adversely affected

Confidential VMs include AMD SEV and Intel TDX guests want to allocate a swiotlb buffer 
that is not restricted to low memory in TEE.

Power SVM already supports this, but X86 still faces the same limitation.
Commit: 8ba2ed1be9 ( "swiotlb: add a SWIOTLB_ANY flag to lift the low memory restriction" )

>
>Thx.
>
>--
>Regards/Gruss,
>    Boris.
>
>https://people.kernel.org/tglx/notes-about-netiquette