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Date:   Wed, 29 Nov 2017 08:56:26 +0100
From:   Juergen Gross <>
To:     "Rafael J. Wysocki" <>
Cc:     Linux Kernel Mailing List <>,,
        the arch/x86 maintainers <>,
        Boris Ostrovsky <>,
        "H. Peter Anvin" <>,
        Thomas Gleixner <>,
        Ingo Molnar <>,
        Jonathan Corbet <>,
        "Rafael J. Wysocki" <>,
        Len Brown <>,
        ACPI Devel Maling List <>
Subject: Re: [PATCH 0/3] x86: make rsdp address accessible via boot params

On 28/11/17 22:03, Rafael J. Wysocki wrote:
> On Tue, Nov 28, 2017 at 10:43 AM, Juergen Gross <> wrote:
>> In the non-EFI boot path the ACPI RSDP table is currently found via
>> either EBDA or by searching through low memory for the RSDP magic.
>> This requires the RSDP to be located in the first 1MB of physical
>> memory. Xen PVH guests, however, get the RSDP address via the start of
>> day information block.
>> In order to support an arbitrary RSDP address this patch series adds
>> the physical address of the RSDP to the boot params structure filled
>> by the boot loader. A kernel booted directly in PVH mode can save the
>> RSDP address in the boot params, while a kernel booted in PVH mode via
>> grub can rely on the RSDP address being specified by grub2 (which in
>> turn got the address via the start of day information block from Xen).
>> Juergen Gross (3):
>>   x86/boot: add acpi rsdp address to setup_header
>>   x86/acpi: take rsdp address for boot params if available
>>   x86/xen: supply rsdp address in boot params for pvh guests
>>  Documentation/x86/boot.txt            | 19 +++++++++++++++++++
>>  arch/x86/boot/header.S                |  6 +++++-
>>  arch/x86/include/uapi/asm/bootparam.h |  1 +
>>  arch/x86/xen/enlighten_pvh.c          |  2 ++
>>  drivers/acpi/osl.c                    |  8 ++++++++
>>  5 files changed, 35 insertions(+), 1 deletion(-)
>> --
> Is this going to work with all existing setups?

I think so, yes.

In EFI environment this doesn't matter, direct PVH boot is working,
grub2 support is optional (without grub2 support things are working as

I'm already writing grub2 patches to support booting in PVH environment.
Those were the reason I need this patch set, as otherwise Xen would have
to put the RSDP into low memory which is limiting the guest's ability to
use large page mappings for all its memory.

Additionally something like this is needed for PVH dom0 support anyway.


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