linux-kernel - Re: [PATCH V12 3/7] dma: add Qualcomm Technologies HIDMA management driver

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Date:	Fri, 15 Jan 2016 12:16:55 -0500
From:	Sinan Kaya <okaya@...eaurora.org>
To:	Mark Rutland <mark.rutland@....com>
Cc:	dmaengine@...r.kernel.org, timur@...eaurora.org,
	devicetree@...r.kernel.org, cov@...eaurora.org,
	vinod.koul@...el.com, jcm@...hat.com, agross@...eaurora.org,
	arnd@...db.de, linux-arm-msm@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	kvmarm@...ts.cs.columbia.edu, marc.zyngier@....com,
	christoffer.dall@...aro.org, shankerd@...eaurora.org,
	Vikram Sethi <vikrams@...eaurora.org>
Subject: Re: [PATCH V12 3/7] dma: add Qualcomm Technologies HIDMA management
 driver

>>> This doesn't seem to tie into KVM or VFIO, and as far as I can tell
>>> there's no mechanism for associating channels with a particular virtual
>>> address space (i.e. no configuration of an external or internal IOMMU),
>>> nor pinning of guest pages to allow for DMA to occur safely.
>>
>> I'm using VFIO platform driver for this purpose. VFIO platform driver is 
>> capable of assigning any platform device to a guest machine with this driver. 
> 
> Typically VFIO-platform also comes with a corresponding reset driver.
> You don't need one?

The HIDMA channel driver resets the channel before using it. That's why, I never
bothered with writing a reset driver on the hypervisor.

> 
>> You just unbind the HIDMA channel driver from the hypervisor and bind to vfio
>> driver using the very same approach you'd use with PCIe. 
>>
>> Of course, this all assumes the presence of an IOMMU driver on the system. VFIO
>> driver uses the IOMMU driver to create the mappings. 
> 
> No IOMMU was described in the DT binding. It sounds like you'd need an
> optional (not present in the guest) iommus property per-channel

You are right. I missed that part. I'll update the device-tree binding documentation.

> 
>> The mechanism used here is not different from VFIO PCI from user perspective.
>>
>>>
>>> Given that, I'm at a loss as to how this would be used in a hypervisor
>>> context. What am I missing?
>>>
>>> Are there additional patches, or do you have some userspace that works
>>> with this in some limited configuration?
>>
>> No, these are the only patches. We have one patch for the QEMU but from kernel
>> perspective this is it. 
> 
> Do you have a link to that? Seeing it would help to ease my concerns.

The QEMU driver has not been posted yet. As far as I know, it just discovers the memory
resources on the platform object and creates mappings for the guest machine only. 

Shanker Donthineni and Vikram Sethi will post the QEMU patch later.

> 
> Thanks,
> Mark.
> 


-- 
Sinan Kaya
Qualcomm Technologies, Inc. on behalf of Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project