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

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Message-ID: <yq5aikj69kpn.fsf@kernel.org>
Date: Sat, 02 Aug 2025 14:14:20 +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 04/38] tsm: Support DMA Allocation from private
 memory

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

> On Tue, Jul 29, 2025 at 01:53:10PM +0530, Aneesh Kumar K.V wrote:
>> Jason Gunthorpe <jgg@...pe.ca> writes:
>> 
>> > On Mon, Jul 28, 2025 at 07:21:41PM +0530, Aneesh Kumar K.V (Arm) wrote:
>> >> @@ -48,3 +49,12 @@ int set_memory_decrypted(unsigned long addr, int numpages)
>> >>  	return crypt_ops->decrypt(addr, numpages);
>> >>  }
>> >>  EXPORT_SYMBOL_GPL(set_memory_decrypted);
>> >> +
>> >> +bool force_dma_unencrypted(struct device *dev)
>> >> +{
>> >> +	if (dev->tdi_enabled)
>> >> +		return false;
>> >
>> > Is this OK? I see code like this:
>> >
>> > static inline dma_addr_t phys_to_dma_direct(struct device *dev,
>> > 		phys_addr_t phys)
>> > {
>> > 	if (force_dma_unencrypted(dev))
>> > 		return phys_to_dma_unencrypted(dev, phys);
>> > 	return phys_to_dma(dev, phys);
>> >
>> > What are the ARM rules for generating dma addreses?
>> >
>> > 1) Device is T=0, memory is unencrypted, call dma_addr_unencrypted()
>> >    and do "top bit IBA set"
>> >
>> > 2) Device is T=1, memory is encrypted, use the phys_to_dma() normally
>> >
>> > 3) Device it T=1, memory is uncrypted, use the phys_to_dma()
>> >    normally??? Seems odd, I would have guessed the DMA address sould
>> >    be the same as case #1?
>> >
>> > Can you document this in a comment?
>> >
>> 
>> If a device is operating in secure mode (T=1), it is currently assumed
>> that only access to private (encrypted) memory is supported.
>
> No, this is no how the PCI specs were written as far as I
> understand. The XT bit thing is supposed to add more fine grained
> device side control over what memory the DMA can target. T alone does
> not do that.
>
>> It is unclear whether devices would need to perform DMA to shared
>> (unencrypted) memory while operating in this mode, as TLPs with T=1
>> are generally expected to target private memory.
>
> PCI SIG supports it, kernel should support it.
>

Would we also need a separate DMA allocation API for allocating
addresses intended to be shared with the non-secure hypervisor?

Are there any existing drivers in the kernel that already require such
shared allocations, which I could use as a reference?

-aneesh