linux-kernel - Re: [Intel-gfx] [Announcement] 2015-Q3 release of XenGT - a Mediated Graphics Passthrough Solution from Intel

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Date:	Mon, 23 Nov 2015 13:05:47 +0800
From:	Jike Song <jike.song@...el.com>
To:	Alex Williamson <alex.williamson@...hat.com>
CC:	"Tian, Kevin" <kevin.tian@...el.com>,
	"xen-devel@...ts.xen.org" <xen-devel@...ts.xen.org>,
	"igvt-g@...1.01.org" <igvt-g@...1.01.org>,
	"intel-gfx@...ts.freedesktop.org" <intel-gfx@...ts.freedesktop.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"White, Michael L" <michael.l.white@...el.com>,
	"Dong, Eddie" <eddie.dong@...el.com>,
	"Li, Susie" <susie.li@...el.com>,
	"Cowperthwaite, David J" <david.j.cowperthwaite@...el.com>,
	"Reddy, Raghuveer" <raghuveer.reddy@...el.com>,
	"Zhu, Libo" <libo.zhu@...el.com>,
	"Zhou, Chao" <chao.zhou@...el.com>,
	"Wang, Hongbo" <hongbo.wang@...el.com>,
	"Lv, Zhiyuan" <zhiyuan.lv@...el.com>,
	qemu-devel <qemu-devel@...gnu.org>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Gerd Hoffmann <kraxel@...hat.com>
Subject: Re: [Intel-gfx] [Announcement] 2015-Q3 release of XenGT - a Mediated
 Graphics Passthrough Solution from Intel

On 11/21/2015 01:25 AM, Alex Williamson wrote:
> On Fri, 2015-11-20 at 08:10 +0000, Tian, Kevin wrote:
>>
>> Here is a more concrete example:
>>
>> KVMGT doesn't require IOMMU. All DMA targets are already replaced with
>> HPA thru shadow GTT. So DMA requests from GPU all contain HPAs.
>>
>> When IOMMU is enabled, one simple approach is to have vGPU IOMMU
>> driver configure system IOMMU with identity mapping (HPA->HPA). We
>> can't use (GPA->HPA) since GPAs from multiple VMs are conflicting.
>>
>> However, we still have host gfx driver running. When IOMMU is enabled,
>> dma_alloc_*** will return IOVA (drvers/iommu/iova.c) in host gfx driver,
>> which will have IOVA->HPA programmed to system IOMMU.
>>
>> One IOMMU device entry can only translate one address space, so here
>> comes a conflict (HPA->HPA vs. IOVA->HPA). To solve this, vGPU IOMMU
>> driver needs to allocate IOVA from iova.c for each VM w/ vGPU assigned,
>> and then KVMGT will program IOVA in shadow GTT accordingly. It adds
>> one additional mapping layer (GPA->IOVA->HPA). In this way two
>> requirements can be unified together since only IOVA->HPA mapping
>> needs to be built.
>>
>> So unlike existing type1 IOMMU driver which controls IOMMU alone, vGPU
>> IOMMU driver needs to cooperate with other agent (iova.c here) to
>> co-manage system IOMMU. This may not impact existing VFIO framework.
>> Just want to highlight additional work here when implementing the vGPU
>> IOMMU driver.
>
> Right, so the existing i915 driver needs to use the DMA API and calls
> like dma_map_page() to enable translations through the IOMMU.  With
> dma_map_page(), the caller provides a page address (~HPA) and is
> returned an IOVA.  So unfortunately you don't get to take the shortcut
> of having an identity mapping through the IOMMU unless you want to
> convert i915 entirely to using the IOMMU API, because we also can't have
> the conflict that an HPA could overlap an IOVA for a previously mapped
> page.
>
> The double translation, once through the GPU MMU and once through the
> system IOMMU is going to happen regardless of whether we can identity
> map through the IOMMU.  The only solution to this would be for the GPU
> to participate in ATS and provide pre-translated transactions from the
> GPU.  All of this is internal to the i915 driver (or vfio extension of
> that driver) and needs to be done regardless of what sort of interface
> we're using to expose the vGPU to QEMU.  It just seems like VFIO
> provides a convenient way of doing this since you'll have ready access
> to the HVA-GPA mappings for the user.
>
> I think the key points though are:
>
>        * the VFIO type1 IOMMU stores GPA to HVA translations
>        * get_user_pages() on the HVA will pin the page and give you a
>          page
>        * dma_map_page() receives that page, programs the system IOMMU and
>          provides an IOVA
>        * the GPU MMU can then be programmed with the GPA to IOVA
>          translations

Thanks for such a nice example! I'll do my home work and get back to you
shortly :)

>
> Thanks,
> Alex
>

--
Thanks,
Jike
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