linux-kernel - Re: [Xen-devel] [PATCH] xen/x86: Zero out .bss for PV guests

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Date:	Wed, 24 Feb 2016 11:46:54 -0500
From:	Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:	"Luis R. Rodriguez" <mcgrof@...nel.org>,
	David Vrabel <david.vrabel@...rix.com>
Cc:	Andrew Cooper <andrew.cooper3@...rix.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	xen-devel <xen-devel@...ts.xenproject.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Andy Lutomirski <luto@...capital.net>,
	Borislav Petkov <bp@...en8.de>
Subject: Re: [Xen-devel] [PATCH] xen/x86: Zero out .bss for PV guests

On 02/24/2016 11:22 AM, Luis R. Rodriguez wrote:
> On Wed, Feb 24, 2016 at 6:58 AM, David Vrabel <david.vrabel@...rix.com> wrote:
>> Yes.  Can you respin with a commit message explaining?  (Or just provide
>> the message here and I'll fix it up).
> Is there no way to re-use somehow the clear_bss() from bare metal?

xen_start_info lives in .bss and it is initialized in startup assembly code.

We could move it to .data but I'd still want to do this first thing in 
startup_xen(). We may add more code there later that touches something 
in .bss and with delayed section clearing we may blow away that data.

-boris

> This uses a section range:
>
> /* Don't add a printk in there. printk relies on the PDA which is not
> initialized
>     yet. */
> static void __init clear_bss(void)
> {
>          memset(__bss_start, 0,
>                 (unsigned long) __bss_stop - (unsigned long) __bss_start);
> }
>
> Perhaps the section range might be different for PV guests? Or can
> this simply not work even if one added a guest bss section size, or
> would it be too late for PV guests, ie we need to do it in asm on PV
> guests as you did? If so why.
>
>   Luis