linux-kernel - Re: [RFC PATCH v1 04/38] tsm: Support DMA Allocation from private memory

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Message-ID: <aMkcwZpr84eMc4fF@google.com>
Date: Tue, 16 Sep 2025 08:16:01 +0000
From: Mostafa Saleh <smostafa@...gle.com>
To: "Aneesh Kumar K.V" <aneesh.kumar@...nel.org>
Cc: linux-coco@...ts.linux.dev, kvmarm@...ts.linux.dev,
	linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
	aik@....com, lukas@...ner.de, Samuel Ortiz <sameo@...osinc.com>,
	Xu Yilun <yilun.xu@...ux.intel.com>, Jason Gunthorpe <jgg@...pe.ca>,
	Suzuki K Poulose <Suzuki.Poulose@....com>,
	Steven Price <steven.price@....com>,
	Catalin Marinas <catalin.marinas@....com>,
	Marc Zyngier <maz@...nel.org>, Will Deacon <will@...nel.org>,
	Oliver Upton <oliver.upton@...ux.dev>
Subject: Re: [RFC PATCH v1 04/38] tsm: Support DMA Allocation from private
 memory

On Tue, Sep 16, 2025 at 09:45:18AM +0530, Aneesh Kumar K.V wrote:
> Mostafa Saleh <smostafa@...gle.com> writes:
> 
> > Hi Aneesh,
> >
> > On Mon, Jul 28, 2025 at 07:21:41PM +0530, Aneesh Kumar K.V (Arm) wrote:
> >> Currently, we enforce the use of bounce buffers to ensure that memory
> >> accessed by non-secure devices is explicitly shared with the host [1].
> >> However, for secure devices, this approach must be avoided.
> >
> >
> > Sorry this might be a basic question, I just started looking into this.
> > I see that “force_dma_unencrypted” and “is_swiotlb_force_bounce” are only
> > used from DMA-direct, but it seems in your case it involves an IOMMU.
> > How does it influence bouncing in that case?
> >
> 
> With the current patchset, the guest does not have an assigned IOMMU (no
> Stage1 SMMU), so guest DMA operations use DMA-direct.
> 
> For non-secure devices:
>  - Streaming DMA uses swiotlb, which is a shared pool with the hypervisor.
>  - Non-streaming DMA uses DMA-direct, and the attributes of the allocated
>    memory are updated with dma_set_decrypted().
> 
> For secure devices, neither of these mechanisms is needed.

I see, thanks for the explanation!

Thanks,
Mostafa

> 
> -aneesh