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Re: [Qemu-devel] [qemu-s390x] [PATCH v2 2/3] hw/s390x/css: Remove QEMU_P


From: David Hildenbrand
Subject: Re: [Qemu-devel] [qemu-s390x] [PATCH v2 2/3] hw/s390x/css: Remove QEMU_PACKED from struct SenseId
Date: Wed, 26 Sep 2018 10:43:09 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.0

On 26/09/2018 10:36, Thomas Huth wrote:
> On 2018-09-26 10:17, David Hildenbrand wrote:
>> On 26/09/2018 10:09, Thomas Huth wrote:
>>> On 2018-09-26 10:07, David Hildenbrand wrote:
>>>> On 26/09/2018 10:04, David Hildenbrand wrote:
>>>>> On 26/09/2018 09:38, Thomas Huth wrote:
>>>>>> The uint16_t member cu_type of struct SenseId is not naturally aligned,
>>>>>> and since the struct is marked with QEMU_PACKED, this can lead to
>>>>>> unaligned memory accesses - which does not work on architectures like
>>>>>> Sparc. Thus remove the QEMU_PACKED here and rather copy the struct
>>>>>> byte by byte when we do copy_sense_id_to_guest().
>>>>>>
>>>>>> Signed-off-by: Thomas Huth <address@hidden>
>>>>>> ---
>>>>>>  hw/s390x/css.c         | 33 +++++++++++++++++----------------
>>>>>>  include/hw/s390x/css.h |  2 +-
>>>>>>  2 files changed, 18 insertions(+), 17 deletions(-)
>>>>>>
>>>>>> diff --git a/hw/s390x/css.c b/hw/s390x/css.c
>>>>>> index 5a9fe45..0e51b85 100644
>>>>>> --- a/hw/s390x/css.c
>>>>>> +++ b/hw/s390x/css.c
>>>>>> @@ -750,20 +750,20 @@ static void sch_handle_halt_func(SubchDev *sch)
>>>>>>  
>>>>>>  }
>>>>>>  
>>>>>> -static void copy_sense_id_to_guest(SenseId *dest, SenseId *src)
>>>>>> +static void copy_sense_id_to_guest(uint8_t *dest, SenseId *src)
>>>>>>  {
>>>>>>      int i;
>>>>>>  
>>>>>> -    dest->reserved = src->reserved;
>>>>>> -    dest->cu_type = cpu_to_be16(src->cu_type);
>>>>>> -    dest->cu_model = src->cu_model;
>>>>>> -    dest->dev_type = cpu_to_be16(src->dev_type);
>>>>>> -    dest->dev_model = src->dev_model;
>>>>>> -    dest->unused = src->unused;
>>>>>> -    for (i = 0; i < ARRAY_SIZE(dest->ciw); i++) {
>>>>>> -        dest->ciw[i].type = src->ciw[i].type;
>>>>>> -        dest->ciw[i].command = src->ciw[i].command;
>>>>>> -        dest->ciw[i].count = cpu_to_be16(src->ciw[i].count);
>>>>>> +    dest[0] = src->reserved;
>>>>>> +    stw_be_p(dest + 1, src->cu_type);
>>>>>> +    dest[3] = src->cu_model;
>>>>>> +    stw_be_p(dest + 4, src->dev_type);
>>>>>> +    dest[6] = src->dev_model;
>>>>>> +    dest[7] = src->unused;
>>>>>> +    for (i = 0; i < ARRAY_SIZE(src->ciw); i++) {
>>>>>> +        dest[8 + i * 4] = src->ciw[i].type;
>>>>>> +        dest[9 + i * 4] = src->ciw[i].command;
>>>>>> +        stw_be_p(dest + 10 + i * 4, src->ciw[i].count);
>>>>>
>>>>>
>>>>> Not really a fan of this, as we sacrifice readability due to one
>>>>> unaligned member. What about only converting the unaligned members (e.g.
>>>>> cu_type) from uint16_t to uint8_t[2] and adding a comment why this is
>>>>> split. Then the structure is naturally packed.
>>>>>
>>>>> We only have to fixup the places that check cu_type.
>>>>>
>>>>
>>>> Just realized this was basically suggested by Peter. If it would be as
>>>> simple as splitting VMSTATE_UINT16 into two VMSTATE_UINT8 or similar, I
>>>> would prefer that.
>>>
>>> It's not that simple, it would break migration from older versions of
>>> QEMU due to endianness issues then.
>>
>> Migration between different QEMUs (e.g. big to little) is not supported
>> as far as I remember. But my head always hurts when thinking about
>> endianness conversions, so I am pretty sure I am missing something here.
> 
> I was not talking about migration between hosts with different
> endianess, but e.g. migration from a x86 host to a x86 host. If you want
> to send 0x1234, that would be 0x34 0x12 when using a 16-bit value, but
> if you break it up into hi- and low, then it's 0x12 0x34 instead.
> 
> Hmm, actually the migration code seems to properly convert 16-bit values
> to network byte order, so maybe this could even work. But honestly, I
> still think we should avoid QEMU_PACKED as much as possible and better
> fill in the memory in copy_sense_id_to_guest() via a byte array here. As
> we've seen now, QEMU_PACKED can easily result in non-portable code, so
> even if copy_sense_id_to_guest() looks a little bit uglier now than
> before, it's certainly the better and more portable way to do this.
> 
>  Thomas
> 

IMHO something like that looks much better (hope I am not messing up
cu_type)


diff --git a/hw/s390x/css.c b/hw/s390x/css.c
index 5a9fe45ce8..db378f6238 100644
--- a/hw/s390x/css.c
+++ b/hw/s390x/css.c
@@ -750,12 +750,13 @@ static void sch_handle_halt_func(SubchDev *sch)

 }

-static void copy_sense_id_to_guest(SenseId *dest, SenseId *src)
+static void copy_sense_id_to_guest(SenseIdPacked *dest, SenseId *src)
 {
     int i;

     dest->reserved = src->reserved;
-    dest->cu_type = cpu_to_be16(src->cu_type);
+    /* avoid unaligned accesses */
+    dest->cu_type[0] = cpu_to_be16(src->cu_type) >> 8;
+    dest->cu_type[1] = cpu_to_be16(src->cu_type);
     dest->cu_model = src->cu_model;
     dest->dev_type = cpu_to_be16(src->dev_type);
     dest->dev_model = src->dev_model;
@@ -1044,7 +1045,7 @@ static int css_interpret_ccw(SubchDev *sch, hwaddr
ccw_addr,
         break;
     case CCW_CMD_SENSE_ID:
     {
-        SenseId sense_id;
+        SenseIdPacked sense_id;

         copy_sense_id_to_guest(&sense_id, &sch->id);
         /* Sense ID information is device specific. */
diff --git a/include/hw/s390x/css.h b/include/hw/s390x/css.h
index 9da5912921..592640f4dd 100644
--- a/include/hw/s390x/css.h
+++ b/include/hw/s390x/css.h
@@ -38,6 +38,17 @@ typedef struct CIW {
     uint16_t count;
 } QEMU_PACKED CIW;

+/* Same as SenseID but naturally packed (what the guest wants) */
+typedef struct SenseIdPacked {
+    uint8_t reserved;
+    uint8_t cu_type[2];
+    uint8_t cu_model;
+    uint16_t dev_type;
+    uint8_t dev_model;
+    uint8_t unused;
+    CIW ciw[MAX_CIWS];
+} SenseId;
+
 typedef struct SenseId {
     /* common part */
     uint8_t reserved;        /* always 0x'FF' */
@@ -48,7 +59,7 @@ typedef struct SenseId {
     uint8_t unused;          /* padding byte */
     /* extended part */
     CIW ciw[MAX_CIWS];       /* variable # of CIWs */
-} QEMU_PACKED SenseId;
+} SenseId;

 /* Channel measurements, from linux/drivers/s390/cio/cmf.c. */
 typedef struct CMB {


-- 

Thanks,

David / dhildenb



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