Re: [Qemu-devel] [RFC] vhost-user: introduce F_NEED_ALL_IOTLB protocol feature

[Michael S. Tsirkin]

On Thu, Apr 12, 2018 at 11:43:41AM +0800, Jason Wang wrote:
> 
> 
> On 2018年04月12日 11:35, Michael S. Tsirkin wrote:
> > > There are more advantages to using request with PASID:
> > > 
> > > You can use hardware support for nesting, having guest supply 1st level
> > > translation and host second level translation.
> > > 
> > > I actually had an idea to do something like this for AMD
> > > and ARM which support nesting even for requests with PASID,
> > > having intel benefit too would be nice.
> > Something else to consider is implementing PRS capability.
> > 
> > 
> > In theory this could then go like this:
> > 
> > - get page request from device
> > - fetch request from VTD page tables
> > - use response to issue a page response message
> > 
> > 
> > This would match the current vhost-user model.
> 
> This requires IOMMU driver can forward this to VFIO and then VFIO can
> forward it to userspace. Looks like a lot of changes and it would be even
> slower than what is proposed in this patch.
> 
> Thanks


It would work better for a static table as only accessed pages would
need to be sent.  Slower for the dynamic case but dynamic case needs
hardware support to work properly in any case.

-- 
MST