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

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Message-ID: <20250802134154.GI26511@ziepe.ca>
Date: Sat, 2 Aug 2025 10:41:54 -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 Sat, Aug 02, 2025 at 02:14:20PM +0530, Aneesh Kumar K.V wrote:
> 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?

The most likely case in the near term is PCI P2P to shared MMIO.

I don't know any way to allocate shared memory in a driver??

At the bare minimum this patch should be documenting the correct
architecture and outlining any gaps in the current implementation.

I also don't really understand what the above code is even
doing.. Isn't the design on ARM that the IPA always encodes the
shared/private in the top bit?

How do we get a shared page that does not already have a phys_addr_t
in the shared IPA? Shouldn't the kernel have switched to the shared
IPA alias when it returned the swiotlb buffer? eg why do we need to do:

#define dma_addr_unencrypted(x)		((x) | PROT_NS_SHARED)

At all?

Suzuki ?

Jason