Re: [Xen-devel] Design doc of adding ACPI support for arm64 on Xen - version 2

[Ian Campbell <ian.campbell@xxxxxxxxxx>]

On Tue, 2015-08-11 at 16:11 +0100, Julien Grall wrote:
> On 11/08/15 15:59, Ian Campbell wrote:
> > On Tue, 2015-08-11 at 15:51 +0100, David Vrabel wrote:
> > > On 11/08/15 15:12, Ian Campbell wrote:
> > > > On Fri, 2015-08-07 at 10:11 +0800, Shannon Zhao wrote:
> > > > > 
> > > > [...]
> > > > > 3. Dom0 gets grant table and event channel irq information
> > > > > -----------------------------------------------------------
> > > > > As said above, we assign the hypervisor_id be "XenVMM" to tell 
> > > > > Dom0 
> > > > > that
> > > > > it runs on Xen hypervisor.
> > > > > 
> > > > > For grant table, add two new HVM_PARAMs: 
> > > > > HVM_PARAM_GNTTAB_START_ADDRESS
> > > > > and HVM_PARAM_GNTTAB_SIZE.
> > > > 
> > > > The reason we expose this range is essentially to allow OS authors 
> > > > to 
> > > > take
> > > > a short cut by telling them about an IPA range which is unused, so 
> > > > it 
> > > > is
> > > > available for remapping the grant table into. On x86 there is a BAR 
> > > > on 
> > > > the
> > > > Xen platform PCI device which serves a similar purpose.
> > > > 
> > > > IIRC somebody (perhaps David V, CCd) had proposed at some point to 
> > > > make 
> > > > it
> > > > so that Linux was able to pick such an IPA itself by examining the 
> > > > memory
> > > > map or by some other scheme.
> > > 
> > > PVH in Linux uses ballooned pages which are vmap()'d into a virtually
> > > contiguous region.
> > > 
> > > See xlated_setup_gnttab_pages().
> > 
> > So somewhat more concrete than a proposal then ;-)
> > 
> > I don't see anything there which would be a problem on ARM, so we 
> > should
> > probably go that route there too (at least for ACPI, if not globally 
> > for
> > all ARM guests).
> 
> IIRC we talked about it few months ago and you said that using balloon
> page will split in 4K the 1G/2M mapping done in the stage-2 p2m.

Did I? Odd because I'm also of the opinion that alloc_ballooned_pages
should operate in chunks of 2M at the hypercall layer and keep any
resulting spare 4K pages on a free list to use for future such allocations.

IOW it should avoid such shattering where it can.

> Note that for DOM0 we never give back the ballooned page to Xen. This is
> because we have to respect the 1:1 mapping when DOM0 want to get back
> the page.
> 
> Regards,
> 

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