Re: [Qemu-devel] [RFC v1 2/3] intel_iommu: add 256 bits qi_desc support

[Peter Xu <peterx@redhat.com>]

On Thu, Feb 14, 2019 at 08:22:05AM +0000, Tian, Kevin wrote:
> > From: Peter Xu [mailto:peterx@redhat.com]
> > Sent: Thursday, February 14, 2019 4:13 PM
> > 
> > On Thu, Feb 14, 2019 at 07:35:20AM +0000, Tian, Kevin wrote:
> > > > From: Peter Xu [mailto:peterx@redhat.com]
> > > > Sent: Thursday, February 14, 2019 3:14 PM
> > > >
> > > > >
> > > > > > When 256 bits invalidation descriptor is used, the guest driver
> > > > > > should be responsible to fill in zeros into reserved fields.
> > > > > >
> > > > > > Another question: is val[2] & val[3] used in any place even with
> > > > > > 256bits mode?  From what I see from the spec (chap 6.5.2), all of
> > them
> > > > > > seems to be reserved as zeros, then I don't understand why bother
> > > > > > extending this to 256bits...  Did I miss something?
> > > > > >
> > >
> > > PRQ is extended to carry larger private data which requires 256bits
> > > mode. You can take a look at 7.5.1.1 Page Request Descriptor.
> > 
> > But we are talking about IQ (Invalidation Queue), not PRQ, right?
> > 
> > I see that Page Request Queue seems to always have 256bits descriptors
> > (chap 10.4.32, there is no Descriptor Width field, and I think it
> > means DW==1 always), however the Invalidation Queue seems to support
> > both modes (chap 10.4.23, there is Descriptor Width field, DW==0
> > should be the legacy mode, and DW==1 should be the new mode).  While,
> > none of the invalidate descriptors described in chap 6.5.2 is using
> > the upper 128bits even if 256bits mode is supported.
> > 
> 
> Page Group Response descriptor is composed by software through
> invalidation queue, which needs to carry same private data back
> (as in page request descriptor). it's described in 7.7.1.

I see the point.  Thanks, Kevin.

Regards,

-- 
Peter Xu