RE: [RFC PATCH 0/4] Use 1st-level for DMA remapping in guest

[Tian, Kevin]

> From: Peter Xu [mailto:peterx@xxxxxxxxxx]
> Sent: Wednesday, September 25, 2019 2:57 PM
> 
> On Wed, Sep 25, 2019 at 10:48:32AM +0800, Lu Baolu wrote:
> > Hi Kevin,
> >
> > On 9/24/19 3:00 PM, Tian, Kevin wrote:
> > > > > >       '-----------'
> > > > > >       '-----------'
> > > > > >
> > > > > > This patch series only aims to achieve the first goal, a.k.a using
> > > first goal? then what are other goals? I didn't spot such information.
> > >
> >
> > The overall goal is to use IOMMU nested mode to avoid shadow page
> table
> > and VMEXIT when map an gIOVA. This includes below 4 steps (maybe not
> > accurate, but you could get the point.)
> >
> > 1) GIOVA mappings over 1st-level page table;
> > 2) binding vIOMMU 1st level page table to the pIOMMU;
> > 3) using pIOMMU second level for GPA->HPA translation;
> > 4) enable nested (a.k.a. dual stage) translation in host.
> >
> > This patch set aims to achieve 1).
> 
> Would it make sense to use 1st level even for bare-metal to replace
> the 2nd level?
> 
> What I'm thinking is the DPDK apps - they have MMU page table already
> there for the huge pages, then if they can use 1st level as the
> default device page table then it even does not need to map, because
> it can simply bind the process root page table pointer to the 1st
> level page root pointer of the device contexts that it uses.
> 

Then you need bear with possible page faults from using CPU page
table, while most devices don't support it today. 

Thanks
Kevin