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Date: Sat, 22 Dec 2018 01:34:01 +0800
From: Yu Zhang <yu.c.zhang@linux.intel.com>
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Subject: Re: [Qemu-devel] [PATCH v3 2/2] intel-iommu: extend VTD emulation
 to allow 57-bit IOVA address width.
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To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Eduardo Habkost <ehabkost@redhat.com>, qemu-devel@nongnu.org, Peter Xu <peterx@redhat.com>, Igor Mammedov <imammedo@redhat.com>, Paolo Bonzini <pbonzini@redhat.com>, Richard Henderson <rth@twiddle.net>

On Fri, Dec 21, 2018 at 12:15:26PM -0500, Michael S. Tsirkin wrote:
> On Sat, Dec 22, 2018 at 12:19:20AM +0800, Yu Zhang wrote:
> > > I'd like to avoid poking at the CPU from VTD code. That's all.
> > 
> > OK. So for the short term，how about I remove the check of host cpu, and add a TODO
> > in the comments in vtd_decide_config()? 
> 
> My question would be what happens on an incorrect use?

I believe the vfio_dma_map will return failure for an incorrect use.

> And how does user figure out which values to set?

Well, for now I don't think user can figure out. E.g. if we expose a vIOMMU with
48-bit IOVA capability, yet host only supports 39-bit IOVA, vfio shall return failure,
but the user does not know whose fault it is.

> 
> > As to the check against hardware IOMMU, Peter once had a proposal in
> > http://lists.nongnu.org/archive/html/qemu-devel/2018-11/msg02281.html
> > 
> > Do you have any comment or suggestion on Peter's proposal?
> 
> Sounds reasonable to me. Do we do it on vfio attach or unconditionally?
> 

I guess on vfio attach? Will need more thinking in it.

> 
> > I still do not quite know
> > how to do it for now...
> > 
> > [...]
> > 
> > 
> > B.R.
> > Yu
> 
> 
> 
> -- 
> MST

B.R.
Yu