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

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Message-ID: <20250729143339.GH26511@ziepe.ca>
Date: Tue, 29 Jul 2025 11:33:39 -0300
From: Jason Gunthorpe <jgg@...pe.ca>
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>,
	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, 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.

Jason