Re: [Xen-devel] [PATCH v2] xen/x86: Zero out .bss for PV guests

[Brian Gerst <brgerst@xxxxxxxxx>]

On Wed, Feb 24, 2016 at 10:19 AM, Boris Ostrovsky
<boris.ostrovsky@xxxxxxxxxx> wrote:
> Baremetal kernels clear .bss early in the boot but Xen PV guests don't
> execute that code. They have been able to run without problems because
> Xen domain builder happens to give out zeroed pages. However, since this
> is not really guaranteed, .bss should be explicitly cleared.
>
> (Since we introduce macros for specifying 32- and 64-bit registers we
> can get rid of ifdefs in startup_xen())
>
> Signed-off-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> ---
>  arch/x86/xen/xen-head.S | 29 ++++++++++++++++++++++-------
>  1 file changed, 22 insertions(+), 7 deletions(-)
>
> diff --git a/arch/x86/xen/xen-head.S b/arch/x86/xen/xen-head.S
> index b65f59a..2af87d1 100644
> --- a/arch/x86/xen/xen-head.S
> +++ b/arch/x86/xen/xen-head.S
> @@ -35,16 +35,31 @@
>  #define PVH_FEATURES (0)
>  #endif
>
> -       __INIT
> -ENTRY(startup_xen)
> -       cld
>  #ifdef CONFIG_X86_32
> -       mov %esi,xen_start_info
> -       mov $init_thread_union+THREAD_SIZE,%esp
> +#define REG(register)  %e##register
> +#define WSIZE_SHIFT    2
> +#define STOS           stosl
>  #else
> -       mov %rsi,xen_start_info
> -       mov $init_thread_union+THREAD_SIZE,%rsp
> +#define REG(register)  %r##register
> +#define WSIZE_SHIFT    3
> +#define STOS           stosq
>  #endif
> +
> +       __INIT
> +ENTRY(startup_xen)
> +       cld
> +
> +       /* Clear .bss */
> +       xor REG(ax),REG(ax)
> +       mov $__bss_start,REG(di)
> +       mov $__bss_stop,REG(cx)
> +       sub REG(di),REG(cx)
> +       shr $WSIZE_SHIFT,REG(cx)
> +       rep STOS
> +
> +       mov REG(si),xen_start_info
> +       mov $init_thread_union+THREAD_SIZE,REG(sp)
> +
>         jmp xen_start_kernel
>
>         __FINIT

Use the macros in <asm/asm.h> instead of defining your own.  Also,
xorl %eax,%eax is good for 64-bit too, since the upper bits are
cleared.

--
Brian Gerst

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