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

[Cornelia Huck <cohuck@redhat.com>]

On Thu, 24 Aug 2017 11:05:08 -0400
Farhan Ali <alifm@linux.vnet.ibm.com> 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.

I have not seen this problem on any of the systems I tested on (well, I
would not have merged this if I did...) - RHEL 7 and F26. Do we need a
dynamic allocation to guarantee alignment?