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Date: Wed, 22 Nov 2017 15:45:19 +0100
From: Cornelia Huck <cohuck@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH 2/2] pc-bios/s390-ccw: zero out bss section
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To: Christian Borntraeger <borntraeger@de.ibm.com>
Cc: qemu-devel <qemu-devel@nongnu.org>, qemu-s390x <qemu-s390x@nongnu.org>, Halil Pasic <pasic@linux.vnet.ibm.com>, Alexander Graf <agraf@suse.de>, Richard Henderson <rth@twiddle.net>, Thomas Huth <thuth@redhat.com>

On Wed, 22 Nov 2017 15:26:27 +0100
Christian Borntraeger <borntraeger@de.ibm.com> wrote:

> The QEMU ELF loader does not zero the bss segment.
> This resulted in several bugs, e.g. see
> 
> commit 5d739a4787a5 (s390-ccw.img: Fix sporadic errors with ccw boot image - initialize css)
> commit 6a40fa2669d3 (s390-ccw.img: Initialize next_idx)
> commit 8775d91a0f42 (pc-bios/s390-ccw: Fix problem with invalid virtio-scsi LUN when rebooting)
> 
> Lets fix this once and forever by letting the BIOS zero the bss itself.

s/Lets/Let's/

:)

> 
> Suggested-by: Alexander Graf <agraf@suse.de>
> Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
> ---
>  pc-bios/s390-ccw/start.S | 30 +++++++++++++++++++++++++++---
>  1 file changed, 27 insertions(+), 3 deletions(-)
> 
> diff --git a/pc-bios/s390-ccw/start.S b/pc-bios/s390-ccw/start.S
> index 43f9bd2..eb8d024 100644
> --- a/pc-bios/s390-ccw/start.S
> +++ b/pc-bios/s390-ccw/start.S
> @@ -3,7 +3,7 @@
>   * into the pc-bios directory of qemu.
>   *
>   * Copyright (c) 2013 Alexander Graf <agraf@suse.de>
> - * Copyright 2013 IBM Corp.
> + * Copyright IBM Corp. 2013, 2017
>   *
>   * This work is licensed under the terms of the GNU GPL, version 2 or (at
>   * your option) any later version. See the COPYING file in the top-level
> @@ -13,8 +13,32 @@
>          .globl _start
>  _start:
>  
> -larl	%r15, stack + 0x8000    /* Set up stack */
> -j	main                    /* And call C */
> +	larl   %r15, stack + 0x8000	/* Set up stack */
> +
> +	/* clear bss */
> +	larl %r2, __bss_start
> +	larl %r3, _end
> +	slgr %r3, %r2		/* get sizeof bss */
> +	ltgr	%r3,%r3 	/* bss emtpy? */
> +	jz	done
> +	aghi	%r3,-1
> +	srlg	%r4,%r3,8	/* how many 256 byte chunks? */
> +	ltgr	%r4,%r4
> +	lgr	%r1,%r2
> +	jz	remainder
> +loop:
> +	xc	0(256,%r1),0(%r1)
> +	la	%r1,256(%r1)
> +	brctg	%r4,loop
> +remainder:
> +	larl	%r2,memsetxc
> +	ex	%r3,0(%r2)
> +done:
> +	j      main		/* And call C */
> +
> +memsetxc:
> +	xc	0(1,%r1),0(%r1)
> +
>  
>  /*
>   * void disabled_wait(void)

This looks like the right thing to do.