Re: [Xen-devel] per-domain passthrough/iommu options

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Jan Beulich <JBeulich@xxxxxxxx>
> Sent: 29 July 2019 12:11
> To: Paul Durrant <Paul.Durrant@xxxxxxxxxx>
> Cc: xen-devel (xen-devel@xxxxxxxxxxxxxxxxxxxx) 
> <xen-devel@xxxxxxxxxxxxxxxxxxxx>
> Subject: Re: [Xen-devel] per-domain passthrough/iommu options
> 
> On 26.07.2019 16:54, Paul Durrant wrote:
> >> -----Original Message-----
> >> From: Jan Beulich <JBeulich@xxxxxxxx>
> >> Sent: 26 July 2019 15:34
> >> To: Paul Durrant <Paul.Durrant@xxxxxxxxxx>
> >> Cc: xen-devel (xen-devel@xxxxxxxxxxxxxxxxxxxx) 
> >> <xen-devel@xxxxxxxxxxxxxxxxxxxx>
> >> Subject: Re: [Xen-devel] per-domain passthrough/iommu options
> >>
> >> On 26.07.2019 16:26, Paul Durrant wrote:
> >>>> From: Jan Beulich <JBeulich@xxxxxxxx>
> >>>> Sent: 26 July 2019 15:02
> >>>>
> >>>> On 26.07.2019 15:39, Paul Durrant wrote:
> >>>>> ? I don't think 'private-pt' or 'separate-pt' really capture the fact 
> >>>>> that the page tables match
> >> the
> >>>> P2M. They could just as easily be taken to mean that they are populated 
> >>>> using some other policy.
> >>>>
> >>>> But haven't we recently seen that this fully lock-step population
> >>>> of page tables isn't always correct (or at least desirable)? I
> >>>> vaguely recall other comments to that effect too, from long ago.
> >>>> I'd specifically want to avoid encoding into the interface here
> >>>> that the two are exact mirrors of one another, now and forever.
> >>>
> >>> How do you think we should express it. I agree that it's a bit awkward 
> >>> because of the difference
> >> between HVM and PV domains, but all we can do there really is document it 
> >> I think, so perhaps the
> >> manpage could have something like:
> >>
> >> Sounds reasonable - it at least avoids making the behavior too
> >> much spelled out with regard to the similarity of mappings between
> >> IOMMU page tables and P2M. There's one issue though:
> >>
> >>> 'off'
> >>>
> >>> IOMMU mappings are disabled for the domain and so hardware may not be 
> >>> passed through.
> >>>
> >>> 'sync-pt'
> >>>
> >>> For a PV domain, all writable pages assigned to the domain are identity 
> >>> mapped by MFN in the IOMMU
> >> page tables. Thus a device driver running in the domain may program 
> >> passthrough hardware for DMA
> using
> >> MFN values (i.e. host/machine frame numbers) looked up in its P2M.
> >>> For an HVM domain, all non-foreign RAM pages present in the P2M will be 
> >>> identity mapped by GFN
> >>
> >> Why "identity mapped" here? It's a GFN -> MFN mappingm, isn't it?
> >
> > Yes... it's hard to express. What I want to say, of course, is that device 
> > drivers can use GFNs. Can
> you think of any other form of words that might be better?
> 
> Just omit "identity" in the HVM related sentence?

Ok, I guess that sounds ok. I'll do that.

  Paul

> 
> Jan
_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxxx
https://lists.xenproject.org/mailman/listinfo/xen-devel