RE: [RFC v2 00/22] intel_iommu: expose Shared Virtual Addressing to VM

[Tian, Kevin]

> From: Jason Wang [mailto:address@hidden]
> Sent: Friday, November 1, 2019 3:30 PM
> 
> 
> On 2019/10/31 下午10:07, Liu, Yi L wrote:
> >> From: Jason Wang [mailto:address@hidden]
> >> Sent: Thursday, October 31, 2019 5:33 AM
> >> Subject: Re: [RFC v2 00/22] intel_iommu: expose Shared Virtual
> Addressing to VM
> >>
> >>
> >> On 2019/10/25 下午6:12, Tian, Kevin wrote:
> >>>> From: Jason Wang [mailto:address@hidden]
> >>>> Sent: Friday, October 25, 2019 5:49 PM
> >>>>
> >>>>
> >>>> On 2019/10/24 下午8:34, Liu Yi L wrote:
> >>>>> Shared virtual address (SVA), a.k.a, Shared virtual memory (SVM) on
> >>>>> Intel platforms allow address space sharing between device DMA
> and
> >>>> applications.
> >>>>
> >>>>
> >>>> Interesting, so the below figure demonstrates the case of VM. I
> >>>> wonder how much differences if we compare it with doing SVM
> between
> >>>> device and an ordinary process (e.g dpdk)?
> >>>>
> >>>> Thanks
> >>> One difference is that ordinary process requires only stage-1
> >>> translation, while VM requires nested translation.
> >>
> >> A silly question, then I believe there's no need for VFIO DMA API in this
> case consider
> >> the page table is shared between MMU and IOMMU?
> > Echo Kevin's reply. We use nested translation here. For stage-1, yes, no
> need to use
> > VFIO DMA API. For stage-2, we still use VFIO DMA API to program the
> GPA->HPA
> > mapping to host. :-)
> 
> 
> Cool, two more questions:
> 
> - Can EPT shares its page table with IOMMU L2?

yes, their formats are compatible.

> 
> - Similar to EPT, when GPA->HPA (actually HVA->HPA) is modified by mm,
> VFIO need to use MMU notifier do modify L2 accordingly besides DMA API?
> 

VFIO devices need to pin-down guest memory pages that are mapped
in IOMMU. So notifier is not required since mm won't change the mapping
for those pages.

Thanks
Kevin