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 <al...@linux.vnet.ibm.com> wrote:
>>>>>
>>>>>> Hi,
>>>>>>
>>>>>> There is an issue in QEMU bios which is exposed by commit
>>>>>>
>>>>>> commit 198c0d1f9df8c429502cb744fc26b6ba6e71db74
>>>>>> Author: Halil Pasic <pa...@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 <pa...@linux.vnet.ibm.com>
>>>>>> Message-Id: <20170727154842.23427-1-pa...@linux.vnet.ibm.com>
>>>>>> Reviewed-by: Dong Jia Shi <bjsdj...@linux.vnet.ibm.com>
>>>>>> Signed-off-by: Cornelia Huck <coh...@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
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
