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

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

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