Re: [PATCH v2 1/3] x86/boot: add acpi rsdp address to setup_header

From: Ingo Molnar
Date: Fri Dec 08 2017 - 02:17:19 EST



* Juergen Gross <jgross@xxxxxxxx> wrote:

> Xen PVH guests receive the address of the RSDP table from Xen. In order
> to support booting a Xen PVH guest via grub2 using the standard x86
> boot entry we need a way fro grub2 to pass the RSDP address to the
> kernel.
>
> For this purpose expand the struct setup_header to hold the physical
> address of the RSDP address. Being zero means it isn't specified and
> has to be located the legacy way (searching through low memory or
> EBDA).

s/fro
/for

pedantry:

s/grub2
/Grub2

> Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
> Reviewed-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> ---
> Documentation/x86/boot.txt | 19 +++++++++++++++++++
> arch/x86/boot/header.S | 6 +++++-
> arch/x86/include/uapi/asm/bootparam.h | 1 +
> 3 files changed, 25 insertions(+), 1 deletion(-)
>
> diff --git a/Documentation/x86/boot.txt b/Documentation/x86/boot.txt
> index 5e9b826b5f62..a33c224797e4 100644
> --- a/Documentation/x86/boot.txt
> +++ b/Documentation/x86/boot.txt
> @@ -61,6 +61,13 @@ Protocol 2.12: (Kernel 3.8) Added the xloadflags field and extension fields
> to struct boot_params for loading bzImage and ramdisk
> above 4G in 64bit.
>
> +Protocol 2.13: (Kernel 3.14) Support 32- and 64-bit flags being set in
> + xloadflags to support booting a 64 bit kernel from 32 bit
> + EFI

The changelog should I think declare that we add documentation for the 2.13
protocol iteration as well.

Also, please use a consistent spelling of '32-bit' and '64-bit' in the same
sentence!

> +Field name: acpi_rsdp_addr
> +Type: write
> +Offset/size: 0x268/8
> +Protocol: 2.14+
> +
> + This field can be set by the boot loader to tell the kernel the
> + physical address of the ACPI RSDP table.
> +
> + A value of 0 indicates the kernel should fall back to the standard
> + methods to locate the RSDP (search in EBDA/low memory).

That's not the only method used: the ACPI RSDP address can also be discovered via
efi.rsdp20 and efi.rsdp, both of which appear to be 32-bit values.

> **** THE IMAGE CHECKSUM
>
> diff --git a/arch/x86/boot/header.S b/arch/x86/boot/header.S
> index 850b8762e889..e7184127f309 100644
> --- a/arch/x86/boot/header.S
> +++ b/arch/x86/boot/header.S
> @@ -300,7 +300,7 @@ _start:
> # Part 2 of the header, from the old setup.S
>
> .ascii "HdrS" # header signature
> - .word 0x020d # header version number (>= 0x0105)
> + .word 0x020e # header version number (>= 0x0105)
> # or else old loadlin-1.5 will fail)
> .globl realmode_swtch
> realmode_swtch: .word 0, 0 # default_switch, SETUPSEG
> @@ -558,6 +558,10 @@ pref_address: .quad LOAD_PHYSICAL_ADDR # preferred load addr
> init_size: .long INIT_SIZE # kernel initialization size
> handover_offset: .long 0 # Filled in by build.c
>
> +acpi_rsdp_addr: .quad 0 # 64-bit physical pointer to
> + # ACPI RSDP table, added with
> + # version 2.14

s/pointer to ACPI RSDP table
/pointer to the ACPI RSDP table

Also, a more fundamental question: why doesn't Xen use EFI to hand over hardware
configuration details?

Thanks,

Ingo