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

[Andrew Cooper]

On 24/02/16 15:19, Boris Ostrovsky 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)

If we are nitpicking,

This should be xor %eax, %eax even in 64bit.  Functionally identical,
and shorter to encode.

~Andrew