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

[Peter Xu <peterx@redhat.com>]

On Wed, Sep 25, 2019 at 08:02:23AM +0000, Tian, Kevin wrote:
> > From: Peter Xu [mailto:peterx@redhat.com]
> > Sent: Wednesday, September 25, 2019 3:45 PM
> > 
> > On Wed, Sep 25, 2019 at 07:21:51AM +0000, Tian, Kevin wrote:
> > > > From: Peter Xu [mailto:peterx@redhat.com]
> > > > 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.
> > 
> > Right, I was just thinking aloud.  After all neither do we have IOMMU
> > hardware to support 1st level (or am I wrong?)...  It's just that when
> 
> You are right. Current VT-d supports only 2nd level.
> 
> > the 1st level is ready it should sound doable because IIUC PRI should
> > be always with the 1st level support no matter on IOMMU side or the
> > device side?
> 
> No. PRI is not tied to 1st or 2nd level. Actually from device p.o.v, it's
> just a protocol to trigger page fault, but the device doesn't care whether
> the page fault is on 1st or 2nd level in the IOMMU side. The only
> relevant part is that a PRI request can have PASID tagged or cleared.
> When it's tagged with PASID, the IOMMU will locate the translation
> table under the given PASID (either 1st or 2nd level is fine, according
> to PASID entry setting). When no PASID is included, the IOMMU locates
> the translation from default entry (e.g. PASID#0 or any PASID contained
> in RID2PASID in VT-d).
> 
> Your knowledge happened to be correct in deprecated ECS mode. At
> that time, there is only one 2nd level per context entry which doesn't
> support page fault, and there is only one 1st level per PASID entry which
> supports page fault. Then PRI could be indirectly connected to 1st level,
> but this just changed with new scalable mode.
> 
> Another note is that the PRI capability only indicates that a device is
> capable of handling page faults, but not that a device can tolerate
> page fault for any of its DMA access. If the latter is fasle, using CPU 
> page table for DPDK usage is still risky (and specific to device behavior)
> 
> > 
> > I'm actually not sure about whether my understanding here is
> > correct... I thought the pasid binding previously was only for some
> > vendor kernel drivers but not a general thing to userspace.  I feel
> > like that should be doable in the future once we've got some new
> > syscall interface ready to deliver 1st level page table (e.g., via
> > vfio?) then applications like DPDK seems to be able to use that too
> > even directly via bare metal.
> > 
> 
> using 1st level for userspace is different from supporting DMA page
> fault in userspace. The former is purely about which structure to
> keep the mapping. I think we may do the same thing for both bare
> metal and guest (using 2nd level only for GPA when nested is enabled
> on the IOMMU). But reusing CPU page table for userspace is more
> tricky. :-)

Yes I should have mixed up the 1st level page table and PRI a bit, and
after all my initial question should be irrelevant to this series as
well so it's already a bit out of topic (sorry for that).

And, thanks for explaining these. :)

-- 
Peter Xu