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Violators will be prosecuted for from ; Thu, 24 Aug 2017 11:53:27 -0400 References: <20170824171355.29d1ec32.cohuck@redhat.com> <30333b41-508d-f9a3-ac2d-4830a7791475@redhat.com> <34f0f307-d16f-7c76-1589-fa06252f6d09@linux.vnet.ibm.com> From: Farhan Ali Date: Thu, 24 Aug 2017 11:53:23 -0400 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Message-Id: Subject: Re: [Qemu-devel] S390 bios breaks in qemu 2.10.rc3 List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Thomas Huth , Halil Pasic , Cornelia Huck Cc: qemu-devel@nongnu.org, Christian Borntraeger , "Collin L. Walling" , David Hildenbrand 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 wrote: >>>> >>>>> Hi, >>>>> >>>>> There is an issue in QEMU bios which is exposed by commit >>>>> >>>>> commit 198c0d1f9df8c429502cb744fc26b6ba6e71db74 >>>>> Author: Halil Pasic >>>>> 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 >>>>> Message-Id: <20170727154842.23427-1-pasic@linux.vnet.ibm.com> >>>>> Reviewed-by: Dong Jia Shi >>>>> Signed-off-by: Cornelia Huck >>>>> >>>>> >>>>> 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