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

[Halil Pasic]

On 08/24/2017 05:35 PM, Thomas Huth wrote:
> On 24.08.2017 17:13, Cornelia Huck wrote:
>> On Thu, 24 Aug 2017 11:05:08 -0400
>> Farhan Ali <address@hidden> wrote:
>>
>>> Hi,
>>>
>>> There is an issue in QEMU bios which is exposed by commit
>>>
>>> commit 198c0d1f9df8c429502cb744fc26b6ba6e71db74
>>> Author: Halil Pasic <address@hidden>
>>> 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 <address@hidden>
>>>      Message-Id: <address@hidden>
>>>      Reviewed-by: Dong Jia Shi <address@hidden>
>>>      Signed-off-by: Cornelia Huck <address@hidden>
>>>
>>>
>>> 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?
> 
> I guess the problem is the __attribute__((packed)) here - AFAIK GCC then
> sometimes assumes that these structs can be byte-aligned. Does it work
> if you remove the __attribute__((packed)) here? If yes, I think that
> would be a valid fix, since there should not be any padding in this
> struct at all (and if you're afraid, you could add an
> assert(sizeof(struct ccw1) == 8) somewhere).
> 
>  Thomas
> 
> 

I don't think this packed is doing us any good. But even
with the packed removed I not sure we would end up being
8 byte aligned (dobuleword). Wouldn't it be just 4 byte
aligned (according to the ELF ABI supplement for s390)
as the most strictly aligned member is __u32?

Halil