Re: [PATCH v2 00/15] vfio: expose virtual Shared Virtual Addressing to VMs

[Stefan Hajnoczi <stefanha@gmail.com>]

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On Mon, Jun 15, 2020 at 12:39:40PM +0000, Liu, Yi L wrote:
> > From: Stefan Hajnoczi <stefanha@gmail.com>
> > Sent: Monday, June 15, 2020 6:02 PM
> > 
> > On Thu, Jun 11, 2020 at 05:15:19AM -0700, Liu Yi L wrote:
> > > Shared Virtual Addressing (SVA), a.k.a, Shared Virtual Memory (SVM) on
> > > Intel platforms allows address space sharing between device DMA and
> > > applications. SVA can reduce programming complexity and enhance security.
> > >
> > > This VFIO series is intended to expose SVA usage to VMs. i.e. Sharing
> > > guest application address space with passthru devices. This is called
> > > vSVA in this series. The whole vSVA enabling requires QEMU/VFIO/IOMMU
> > > changes. For IOMMU and QEMU changes, they are in separate series (listed
> > > in the "Related series").
> > >
> > > The high-level architecture for SVA virtualization is as below, the key
> > > design of vSVA support is to utilize the dual-stage IOMMU translation (
> > > also known as IOMMU nesting translation) capability in host IOMMU.
> > >
> > >
> > >     .-------------.  .---------------------------.
> > >     |   vIOMMU    |  | Guest process CR3, FL only|
> > >     |             |  '---------------------------'
> > >     .----------------/
> > >     | PASID Entry |--- PASID cache flush -
> > >     '-------------'                       |
> > >     |             |                       V
> > >     |             |                CR3 in GPA
> > >     '-------------'
> > > Guest
> > > ------| Shadow |--------------------------|--------
> > >       v        v                          v
> > > Host
> > >     .-------------.  .----------------------.
> > >     |   pIOMMU    |  | Bind FL for GVA-GPA  |
> > >     |             |  '----------------------'
> > >     .----------------/  |
> > >     | PASID Entry |     V (Nested xlate)
> > >     '----------------\.------------------------------.
> > >     |             |   |SL for GPA-HPA, default domain|
> > >     |             |   '------------------------------'
> > >     '-------------'
> > > Where:
> > >  - FL = First level/stage one page tables
> > >  - SL = Second level/stage two page tables
> > 
> > Hi,
> > Looks like an interesting feature!
> 
> thanks for the interest. Stefan :-)
> 
> > To check I understand this feature: can applications now pass virtual
> > addresses to devices instead of translating to IOVAs?
> 
> yes, application could pass virtual addresses to device directly. As
> long as the virtual address is mapped in cpu page table, then IOMMU
> would get it translated to physical address.
> 
> > If yes, can guest applications restrict the vSVA address space so the
> > device only has access to certain regions?
> 
> do you mean restrict the access of certain virtual address regions of
> guest application ? or certain guest memory? :-)

Your reply below answered my question. I was wondering if applications
can protect parts of their virtual memory space that should not be
accessed by the device. It makes sense that there is a trade-off to
simplify the programming model and performance might also be better if
the application doesn't need to DMA map/unmap buffers frequently.

Stefan

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