linux-kernel - Re: [RFC PATCH v1 10/38] iommufd/vdevice: Add TSM map ioctl

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Message-ID: <yq5awm7r9yu4.fsf@kernel.org>
Date: Tue, 29 Jul 2025 14:07:55 +0530
From: Aneesh Kumar K.V <aneesh.kumar@...nel.org>
To: Jason Gunthorpe <jgg@...pe.ca>
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>,
	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 10/38] iommufd/vdevice: Add TSM map ioctl

Jason Gunthorpe <jgg@...pe.ca> writes:

> On Mon, Jul 28, 2025 at 07:21:47PM +0530, Aneesh Kumar K.V (Arm) wrote:
>> With passthrough devices, we need to make sure private memory is
>> allocated and assigned to the secure guest before we can issue the DMA.
>> For ARM RMM, we only need to map and the secure SMMU management is
>> internal to RMM. For shared IPA, vfio/iommufd DMA MAP/UNMAP interface
>> does the equivalent
>
> I'm not really sure what this is about? It is about getting KVM to pin
> all the memory and commit it to the RMM so it can be used for DMA?
>

That is correct.

>
> But it looks really strange to have an iommufd ioctl that just calls a
> KVM function. Feeling this should be a KVM function, or a guestmfd
> behavior??
>
This functionality is equivalent to `IOMMU_IOAS_MAP`, but in the
presence of firmware like RMM, we also need to supply the realm
descriptor associated with the KVM instance.

Initially, I attempted to handle this within the `map_pages` callback in
`iommu_domain_ops`, but that path lacks any awareness of the associated
KVM context, making it unsuitable for this purpose.


> I was kind of thinking it would be nice to have a guestmemfd mode that
> was "pinned", meaning the memory is allocated and remains almost
> always mapped into the TSM's page tables automatically. VFIO using
> guests would set things this way.
>

We need to allocate and free these pages dynamically as they are
converted between private and shared states.

-aneesh