Re: [Qemu-devel] [RFC Design Doc v3] Enable Shared Virtual Memory feature in pass-through scenarios

[Raj, Ashok]

On Wed, Mar 01, 2017 at 04:09:38PM -0500, Konrad Rzeszutek Wilk wrote:
> .snip..
> > 
> > No. SVM is purely about sharing CPU address space with device. Command
> > submission is still through kernel driver which controls rings (with SVM 
> > then
> > you can put VA into those commands). There are other vendor specific 
> > features to enable direct user space submission which is orthogonal to SVM.
> 
> Apologies for my ignorance but how is this beneficial? As in
> currently you would put in bus addresses on the ring, but now you
> can put VA addresses.
> 
> The obvious benefit I see is that you omit the DMA ops which means there is
> less of 'lookup' (VA->bus address) in software - but I would have thought this
> would be negligible performance impact? And now the IOMMU alongside with
> the CPU would do this lookup.
> 
> Or are there some other improvements in this?
Other benefits include,

- Application can simply pass its pointers to the SVM capable devices. which
  means no memory registration overhead to get IO Virtual Addresses. 
- No need to pin memory for DMA, since the devices can handle faults and
  can request pages to be paged-in on demand.
> 
> > 
> > Thanks
> > Kevin
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