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 <th...@redhat.com> >>>>>> --- >>>>>> 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