Re: [PATCH v2] scsi: 3w-9xxx: Fix endianness issues found by sparse

[Samuel Holland]

On 8/3/20 9:02 AM, Arnd Bergmann wrote:
> On Mon, Aug 3, 2020 at 5:42 AM Samuel Holland <samuel@xxxxxxxxxxxx> wrote:
>> On 7/31/20 2:29 AM, Arnd Bergmann wrote:
>>> On Fri, Jul 31, 2020 at 12:07 AM Samuel Holland <samuel@xxxxxxxxxxxx> wrote:
>>>>
>>>> The main issue observed was at the call to scsi_set_resid, where the
>>>> byteswapped parameter would eventually trigger the alignment check at
>>>> drivers/scsi/sd.c:2009. At that point, the kernel would continuously
>>>> complain about an "Unaligned partial completion", and no further I/O
>>>> could occur.
>>>>
>>>> This gets the controller working on big endian powerpc64.
>>>>
>>>> Signed-off-by: Samuel Holland <samuel@xxxxxxxxxxxx>
>>>> ---
>>>>
>>>> Changes since v1:
>>>>  - Include changes to use __le?? types in command structures
>>>>  - Use an object literal for the intermediate "schedulertime" value
>>>>  - Use local "error" variable to avoid repeated byte swapping
>>>>  - Create a local "length" variable to avoid very long lines
>>>>  - Move byte swapping to TW_REQ_LUN_IN/TW_LUN_OUT to avoid long lines
>>>>
>>>
>>> Looks much better, thanks for the update. I see one more issue here
>>>>  /* Command Packet */
>>>>  typedef struct TW_Command {
>>>> -       unsigned char opcode__sgloffset;
>>>> -       unsigned char size;
>>>> -       unsigned char request_id;
>>>> -       unsigned char unit__hostid;
>>>> +       u8      opcode__sgloffset;
>>>> +       u8      size;
>>>> +       u8      request_id;
>>>> +       u8      unit__hostid;
>>>>         /* Second DWORD */
>>>> -       unsigned char status;
>>>> -       unsigned char flags;
>>>> +       u8      status;
>>>> +       u8      flags;
>>>>         union {
>>>> -               unsigned short block_count;
>>>> -               unsigned short parameter_count;
>>>> +               __le16  block_count;
>>>> +               __le16  parameter_count;
>>>>         } byte6_offset;
>>>>         union {
>>>>                 struct {
>>>> -                       u32 lba;
>>>> -                       TW_SG_Entry sgl[TW_ESCALADE_MAX_SGL_LENGTH];
>>>> -                       dma_addr_t padding;
>>>> +                       __le32          lba;
>>>> +                       TW_SG_Entry     sgl[TW_ESCALADE_MAX_SGL_LENGTH];
>>>> +                       dma_addr_t      padding;
>>>
>>>
>>> The use of dma_addr_t here seems odd, since this is neither endian-safe nor
>>> fixed-length. I see you replaced the dma_addr_t in TW_SG_Entry with
>>> a variable-length fixed-endian word. I guess there is a chance that this is
>>> correct, but it is really confusing. On top of that, it seems that there is
>>> implied padding in the structure when built with a 64-bit dma_addr_t
>>> on most architectures but not on x86-32 (which uses 32-bit alignment for
>>> 64-bit integers). I don't know what the hardware definition is for TW_Command,
>>> but ideally this would be expressed using only fixed-endian fixed-length
>>> members and explicit padding.
>>
>> All of the command structures are packed, due to the "#pragma pack(1)" earlier
>> in the file. So alignment is not an issue. This dma_addr_t member _is_ the
>> explicit padding to make sizeof(TW_Command) -
>> sizeof(TW_Command.byte8_offset.{io,param}.sgl) equal TW_COMMAND_SIZE * 4. And
>> indeed the structure is expected to be a different size depending on
>> sizeof(dma_addr_t).
> 
> Ah, so only the first few members are accessed by hardware and the
> last union is only accessed by the OS then? In that case I suppose it is
> all fine, but I would also suggest removing the "#pragma packed"
> to get somewhat more efficient access on systems that have  problems
> with misaligned accesses.

I don't know what part the hardware accesses; everything I know about the
hardware comes from reading the driver.

The problem with removing the "#pragma pack(1)" is that the structure is
inherently misaligned: byte8_offset.io.sgl starts at offset 12, but it may begin
with a __le64.

Regards,
Samuel