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

[Eric Farman <farman@linux.ibm.com>]

On 11/20/20 3:26 AM, Janosch Frank wrote:
> On 11/19/20 5:57 PM, 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>
> 
> Seems like I should have been more specific with my LGTM:
> Reviewed-by: Janosch Frank <frankja@de.ibm.com>

Thanks, Janosch. I didn't want to put words in your acronym. :)

> 
>> ---
>>   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 */
>>
>