Re: [Xen-devel] [PATCH v2 19/21] xen/arm: Reserve region in guest memory for device passthrough

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Wed, 6 Aug 2014, Julien Grall wrote:
> On 08/06/2014 05:45 PM, Stefano Stabellini wrote:
> > On Wed, 6 Aug 2014, Stefano Stabellini wrote:
> >> On Wed, 6 Aug 2014, Julien Grall wrote:
> >>> Hi Stefano,
> >>>
> >>> On 08/06/2014 05:27 PM, Stefano Stabellini wrote:
> >>>> On Thu, 31 Jul 2014, Julien Grall wrote:
> >>>>> This region will be split by the toolstack to allocate MMIO range for 
> >>>>> eac
> >>>>> device.
> >>>>>
> >>>>> For now only reserve a 768MB region, this should be enought to 
> >>>>> passthrough
> >>>>> multiple device at the same time.
> >>>>>
> >>>>> Signed-off-by: Julien Grall <julien.grall@xxxxxxxxxx>
> >>>>>
> >>>>> ---
> >>>>>
> >>>>> I can't allocate easily a range of 1GB in the current layout without 
> >>>>> shrink
> >>>>> the first RAM bank. So I decided to use only 768MB. If in the future we 
> >>>>> need
> >>>>> more, then we will have to choose at runtime the guest layout.
> >>>>>
> >>>>>     Changes in v2:
> >>>>>         - Bump the reserved region from 512MB to 768MB
> >>>>
> >>>>
> >>>> Better than nothing.
> >>>> Would it be possible to add a second region > 4GB? Maybe after RAM?
> >>>> How hard would be to handle that?
> >>>
> >>> It won't work if the guest is using short-descriptor translation table
> >>> format because only 32 bits address will be supported.
> >>
> >> I know.
> > 
> > However it would make things easier if you want to assign a device with
> > a large MMIO region to a guest that can handle it.
> 
> Last time you told me about graphics card. But we don't have any board
> with graphic card and IOMMU supported by Xen.
> In any case the developer will have to modify Xen to add the new
> platform. It won't hurt to add a new region at this time.

I understand your concerns and in theory you are right, but in practice
it would be much easier to add it now than then. The new guy won't know
anything about device passthrough and libxl. While at the moment it is
all still new and fresh in our minds.

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