Re: [Xen-devel] [PATCH v2] libxl: put RSDP for PVH guest near 4GB

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Wed, Jan 24, 2018 at 10:42:39AM +0000, George Dunlap wrote:
> On Wed, Jan 24, 2018 at 2:41 AM, Boris Ostrovsky
> <boris.ostrovsky@xxxxxxxxxx> wrote:
> > On 01/18/2018 05:33 AM, Wei Liu wrote:
> >> On Thu, Jan 18, 2018 at 11:31:32AM +0100, Juergen Gross wrote:
> >>> Wei,
> >>>
> >>> On 01/12/17 15:14, Juergen Gross wrote:
> >>>> Instead of locating the RSDP table below 1MB put it just below 4GB
> >>>> like the rest of the ACPI tables in case of PVH guests. This will
> >>>> avoid punching more holes than necessary into the memory map.
> >>>>
> >>>> Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
> >>>> Acked-by: Wei Liu <wei.liu2@xxxxxxxxxx>
> >>> Mind applying this one?
> >> Don't worry, it is in my queue.
> >>
> >> Will come to this and other patches I accumulated soon.
> >>
> >> Wei.
> >
> > This requires kernel changes, doesn't it?
> >
> > https://lists.xenproject.org/archives/html/xen-devel/2017-12/msg00714.html
> >
> > And this series apparently never made it to the tree.
> >
> > PVH guests are broken now on staging.
> 
> And the Linux side of PVH is officially supported now, right?  Which
> means we need to have some way of being backwards compatible with
> kernels without that patch, even if it does get  backported.

Linux had a bug that on PVH it expected to find the RSDP at the low
1MiB. The PVHv2 boot protocol has _never_ mentioned that the RSDP
should be below 1MiB. There's just one comment in the start_info
header that says:

"Xen on x86 will always try to place all the data below the 4GiB
boundary."

And the RSDP address must be fetched from the rsdp_paddr field of the
start_info.

I agree that the fact that Linux is broken is bad, but should not
force us to change the protocol. What's more, when booting a PVHv2
Dom0 the RSDP _must_ not be searched in the low 1MiB, or else Dom0 is
going to find the firmware provided RSDP and everything is going to
blow up.

IMO, Linux must be fixed and the patch backported up to 4.11.

Roger.

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