Re: [Qemu-devel] S390 bios breaks in qemu 2.10.rc3

[Farhan Ali <alifm@linux.vnet.ibm.com>]

On 08/24/2017 02:33 PM, Christian Borntraeger wrote:
> Just to understand the urgency. Is the problem happening with the BIOS that is shipped in the
> upstream git tree, or a self-built one?
>

The bios image upstream works fine, but if someone tries to recompile 
the bios and use that, it breaks.


>
> On 08/24/2017 05:05 PM, Farhan Ali wrote:
>> Hi,
>>
>> There is an issue in QEMU bios which is exposed by commit
>>
>> commit 198c0d1f9df8c429502cb744fc26b6ba6e71db74
>> Author: Halil Pasic <pasic@linux.vnet.ibm.com>
>> Date:   Thu Jul 27 17:48:42 2017 +0200
>>
>>     s390x/css: check ccw address validity
>>
>>     According to the PoP channel command words (CCW) must be doubleword
>>     aligned and 31 bit addressable for format 1 and 24 bit addressable for
>>     format 0 CCWs.
>>
>>     If the channel subsystem encounters a ccw address which does not satisfy
>>     this alignment requirement a program-check condition is recognised.
>>
>>     The situation with 31 bit addressable is a bit more complicated: both the
>>     ORB and a format 1 CCW TIC hold the address of (the rest of) the channel
>>     program, that is the address of the next CCW in a word, and the PoP
>>     mandates that bit 0 of that word shall be zero -- or a program-check
>>     condition is to be recognized -- and does not belong to the field holding
>>     the ccw address.
>>
>>     Since in code the corresponding fields span across the whole word (unlike
>>     in PoP where these are defined as 31 bit wide) we can check this by
>>     applying a mask. The 24 addressable case isn't affecting TIC because the
>>     address is composed of a halfword and a byte portion (no additional zero
>>     bit requirements) and just slightly complicates the ORB case where also
>>     bits 1-7 need to be zero.
>>
>>     The same requirements (especially n-bit addressability) apply to the
>>     ccw addresses generated while chaining.
>>
>>     Let's make our CSS implementation follow the AR more closely.
>>
>>     Signed-off-by: Halil Pasic <pasic@linux.vnet.ibm.com>
>>     Message-Id: <20170727154842.23427-1-pasic@linux.vnet.ibm.com>
>>     Reviewed-by: Dong Jia Shi <bjsdjshi@linux.vnet.ibm.com>
>>     Signed-off-by: Cornelia Huck <cohuck@redhat.com>
>>
>>
>> It looks like the bios does not create a double word aligned CCW. Looking at the bios code we the CCW1 struct is not aligned
>>
>> /* channel command word (type 1) */
>> struct ccw1 {
>>     __u8 cmd_code;
>>     __u8 flags;
>>     __u16 count;
>>     __u32 cda;
>> } __attribute__ ((packed));
>>
>> and it looks like the compiler does not guarantee a doubleword alignment.
>>
>> The weird thing about it is I see it break in one of my system and works fine in another system. Trying a simple fix of aligning the struct also doesn't seem to work all the time.
>>
>>
>> Thanks
>> Farhan
>>