linux-kernel - Re: [PATCH v2 3/3] x86/xen: supply rsdp address in boot params for pvh guests

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Date:   Fri, 8 Dec 2017 08:22:16 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Juergen Gross <jgross@...e.com>
Cc:     linux-kernel@...r.kernel.org, xen-devel@...ts.xenproject.org,
        x86@...nel.org, boris.ostrovsky@...cle.com, hpa@...or.com,
        tglx@...utronix.de, mingo@...hat.com, corbet@....net,
        rjw@...ysocki.net, lenb@...nel.org, linux-acpi@...r.kernel.org
Subject: Re: [PATCH v2 3/3] x86/xen: supply rsdp address in boot params for
 pvh guests


* Juergen Gross <jgross@...e.com> wrote:

> When booted via the special PVH entry save the RSDP address set in the
> boot information block in struct boot_params. This will enable Xen to
> locate the RSDP at an arbitrary address.
> 
> Set the boot loader version to 2.14 (0x020e) replacing the wrong 0x0212
> which should have been 0x020c.
> 
> Signed-off-by: Juergen Gross <jgross@...e.com>
> ---
> V2: set bootloader version to 2.14 (Roger Pau Monné)
> ---
>  arch/x86/xen/enlighten_pvh.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/xen/enlighten_pvh.c b/arch/x86/xen/enlighten_pvh.c
> index 436c4f003e17..036e3a5f284a 100644
> --- a/arch/x86/xen/enlighten_pvh.c
> +++ b/arch/x86/xen/enlighten_pvh.c
> @@ -68,9 +68,12 @@ static void __init init_pvh_bootparams(void)
>  	 *
>  	 * Version 2.12 supports Xen entry point but we will use default x86/PC
>  	 * environment (i.e. hardware_subarch 0).
> +	 * The RSDP address is available from version 2.14 on.
>  	 */
> -	pvh_bootparams.hdr.version = 0x212;
> +	pvh_bootparams.hdr.version = 0x20e;

While 0x212 was "obvious" to read but totally wrong, it would be less fragile and 
more readable if the version was generated as something like:

	pvh_bootparams.hdr.version = (2 << 8) | 14;

similar to how it's written in other cases:

>  	pvh_bootparams.hdr.type_of_loader = (9 << 4) | 0; /* Xen loader */

Also, shouldn't the 0x212 fix be a separate patch, Cc: stable? The bug appears to 
have been introduced at around v4.12.

Thanks,

	Ingo