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

[Halil Pasic]

On 08/24/2017 05:53 PM, Farhan Ali wrote:
> 
> 
> 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 <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).
>>>
>>> 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

That sounds mysterious. Could you debug that with your colleague
and verify that the problem is indeed the alignment? If it
is I would have to re-read the gcc doc because I can't think of
anything else than a bug either in my understanding or in their product.

A stupid question is he testing with the current master?

Halil