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).
Thomas
