Re: [PATCH 1/2] pc-bios: s390x: Ensure Read IPL memory is clean

[Thomas Huth]

On 19/11/2020 17.57, Eric Farman wrote:
> If, for example, we boot off a virtio device and chreipl to a vfio-ccw
> device, the space at lowcore will be non-zero. We build a Read IPL CCW
> at address zero, but it will have leftover PSW data that will conflict
> with the Format-0 CCW being generated:
> 
> 0x0: 00080000 80010000
>        ------ Ccw0.cda
>               -- Ccw0.chainData
>                 -- Reserved bits
> 
> The data address will be overwritten with the correct value (0x0), but
> the apparent data chain bit will cause subsequent memory to be used as
> the target of the data store, which may not be where we expect (0x0).
> 
> Clear out this space when we boot from DASD, so that we know it exists
> exactly as we expect.
> 
> Signed-off-by: Eric Farman <farman@linux.ibm.com>
> Reviewed-by: Jason J. Herne <jjherne@linux.ibm.com>
> Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>
> ---
>  pc-bios/s390-ccw/dasd-ipl.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/pc-bios/s390-ccw/dasd-ipl.c b/pc-bios/s390-ccw/dasd-ipl.c
> index 0fc879bb8e..71cbae2f16 100644
> --- a/pc-bios/s390-ccw/dasd-ipl.c
> +++ b/pc-bios/s390-ccw/dasd-ipl.c
> @@ -100,6 +100,9 @@ static void make_readipl(void)
>  {
>      Ccw0 *ccwIplRead = (Ccw0 *)0x00;
>  
> +    /* Clear out any existing data */
> +    memset(ccwIplRead, 0, sizeof(Ccw0));
> +
>      /* Create Read IPL ccw at address 0 */
>      ccwIplRead->cmd_code = CCW_CMD_READ_IPL;
>      ccwIplRead->cda = 0x00; /* Read into address 0x00 in main memory */

Sounds reasonable.

Reviewed-by: Thomas Huth <thuth@redhat.com>