Re: [Qemu-devel] [RFC PATCH v3 3/3] VFIO Type1 IOMMU change: to support with iommu and without iommu

[Jike Song]

On 05/11/2016 12:02 AM, Neo Jia wrote:
> On Tue, May 10, 2016 at 03:52:27PM +0800, Jike Song wrote:
>> On 05/05/2016 05:27 PM, Tian, Kevin wrote:
>>>> From: Song, Jike
>>>>
>>>> IIUC, an api-only domain is a VFIO domain *without* underlying IOMMU
>>>> hardware. It just, as you said in another mail, "rather than
>>>> programming them into an IOMMU for a device, it simply stores the
>>>> translations for use by later requests".
>>>>
>>>> That imposes a constraint on gfx driver: hardware IOMMU must be disabled.
>>>> Otherwise, if IOMMU is present, the gfx driver eventually programs
>>>> the hardware IOMMU with IOVA returned by pci_map_page or dma_map_page;
>>>> Meanwhile, the IOMMU backend for vgpu only maintains GPA <-> HPA
>>>> translations without any knowledge about hardware IOMMU, how is the
>>>> device model supposed to do to get an IOVA for a given GPA (thereby HPA
>>>> by the IOMMU backend here)?
>>>>
>>>> If things go as guessed above, as vfio_pin_pages() indicates, it
>>>> pin & translate vaddr to PFN, then it will be very difficult for the
>>>> device model to figure out:
>>>>
>>>>    1, for a given GPA, how to avoid calling dma_map_page multiple times?
>>>>    2, for which page to call dma_unmap_page?
>>>>
>>>> --
>>>
>>> We have to support both w/ iommu and w/o iommu case, since
>>> that fact is out of GPU driver control. A simple way is to use
>>> dma_map_page which internally will cope with w/ and w/o iommu
>>> case gracefully, i.e. return HPA w/o iommu and IOVA w/ iommu.
>>> Then in this file we only need to cache GPA to whatever dmadr_t
>>> returned by dma_map_page.
>>>
>>
>> Hi Alex, Kirti and Neo, any thought on the IOMMU compatibility here?
> 
> Hi Jike,
> 
> With mediated passthru, you still can use hardware iommu, but more important
> that part is actually orthogonal to what we are discussing here as we will 
> only
> cache the mapping between <gfn (iova if guest has iommu), (qemu) va>, once we 
> have pinned pages later with the help of above info, you can map it into the
> proper iommu domain if the system has configured so.
>

Hi Neo,

Technically yes you can map a pfn into the proper IOMMU domain elsewhere,
but to find out whether a pfn was previously mapped or not, you have to
track it with another rbtree-alike data structure (the IOMMU driver simply
doesn't bother with tracking), that seems somehow duplicate with the vGPU
IOMMU backend we are discussing here.

And it is also semantically correct for an IOMMU backend to handle both w/
and w/o an IOMMU hardware? :)


--
Thanks,
Jike