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

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

On 08/24/2017 11:50 AM, Thomas Huth wrote:
> On 24.08.2017 17:47, Halil Pasic wrote:
>>
>>
>> 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 <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?
>>>
>>> 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).
>>
>> 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?
>
> True, so that could still be an issue. Looking at the cio.h in the
> kernel, they define the struct like this:
>
> struct ccw1 {
>         __u8  cmd_code;
>         __u8  flags;
>         __u16 count;
>         __u32 cda;
> } __attribute__ ((packed,aligned(8)));
>
> So I guess adding the aligned(8) is the right way to go?
>
>  Thomas
>

This was my initial fix and it works on my system. But for some reason 
this fix does not work on my colleague's system. So I am hesitant about 
submitting this fix