Re: Criteria for dom0_gnttab_start

[Ian Campbell <Ian.Campbell@citrix.com>]

On Tue, 2014-10-21 at 18:14 +0530, Vijay Kilari wrote:
> On Tue, Oct 21, 2014 at 5:44 PM, Ian Campbell <Ian.Campbell@citrix.com> wrote:
> > On Tue, 2014-10-21 at 13:12 +0100, Julien Grall wrote:
> >> On 10/21/2014 01:04 PM, Ian Campbell wrote:
> >> > On Tue, 2014-10-21 at 17:24 +0530, Vijay Kilari wrote:
> >> >>    What is the criteria for setting dom0_gnttab_start and size for a
> >> >> platform.
> >> >
> >> > It simply needs to be an area of the IPA address space which is not
> >> > populated by anything else. Since dom0's MMIO space is mapped 1:1 with
> >> > the host PA address space it seems easiest/safest to find a hole in the
> >> > real physical space and use that.
> >> >
> >> > If there is no such hole available then using the space occupied by some
> >> > device which is not passed to dom0 (e.g. is blacklisted, or used by Xen)
> >> > will suffice.
> 
> Thanks for the inputs.
> 
> Assume arm64 platform has 4GB physical RAM starting from 0x0
> and all MMIO regions are beyond 4GB. For Xen the physical RAM start
> address is specified at 12MB - 4GB
> by UEFI.The first 12MB PA is used by Secure world.
> In such case, can we specify dom0_gnttab_start anywhere within 0 to
> 12MB IPA address?

I think so, yes, since NS world will never be able to use that 12MB. I
assume that the EFI address map (or DTB) marks that region as reserved?

> OR can I reserve some physical address space within 4GB and use it?

You'd have to tell Xen not to use it as normal RAM for dom0, which would
probably involve some hacks in the domain builder, best avoided I think.

Ian.