Re: [PATCH 2/2] xen: add acpi_arch_get_root_pointer() for pvh guests

[Greg KH <gregkh@linuxfoundation.org>]

On Thu, Jan 25, 2018 at 11:04:54AM +0100, Juergen Gross wrote:
> Add acpi_arch_get_root_pointer() for Xen PVH guests to communicate
> the address of the RSDP table given to the kernel via Xen start info.
> 
> This makes the kernel boot again in PVH mode after on recent Xen the
> RSDP was moved to higher addresses. So up to that change it was pure
> luck that the legacy method to locate the RSDP was working when
> running as PVH mode.
> 
> Cc: <stable@vger.kernel.org> # 4.11
> Signed-off-by: Juergen Gross <jgross@suse.com>
> ---
>  arch/x86/xen/enlighten_pvh.c | 15 ++++++++++++---
>  1 file changed, 12 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/xen/enlighten_pvh.c b/arch/x86/xen/enlighten_pvh.c
> index 436c4f003e17..9a5c3a7fe673 100644
> --- a/arch/x86/xen/enlighten_pvh.c
> +++ b/arch/x86/xen/enlighten_pvh.c
> @@ -16,15 +16,24 @@
>  /*
>   * PVH variables.
>   *
> - * xen_pvh and pvh_bootparams need to live in data segment since they
> - * are used after startup_{32|64}, which clear .bss, are invoked.
> + * xen_pvh, pvh_bootparams and pvh_start_info need to live in data segment
> + * since they are used after startup_{32|64}, which clear .bss, are invoked.
>   */
>  bool xen_pvh __attribute__((section(".data"))) = 0;
>  struct boot_params pvh_bootparams __attribute__((section(".data")));
> +struct hvm_start_info pvh_start_info __attribute__((section(".data")));
>  
> -struct hvm_start_info pvh_start_info;
>  unsigned int pvh_start_info_sz = sizeof(pvh_start_info);
>  
> +acpi_physical_address acpi_arch_get_root_pointer(void)
> +{
> +	if (xen_pvh)
> +		return pvh_start_info.rsdp_paddr;
> +
> +	return 0;
> +}
> +EXPORT_SYMBOL_GPL(acpi_arch_get_root_pointer);

Why does this have to be an exported symbol?  Does this code get built
as a module and will the linker somehow go and rewrite the previous call
places with this one if it gets loaded?

thanks,

greg k-h