[Bug 1881552] Re: potential AArch64 ABI bug wrt handling of 128-bit bit-fields

[Thomas Huth]

** Changed in: qemu
       Status: Fix Committed => Fix Released

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https://bugs.launchpad.net/bugs/1881552

Title:
  potential AArch64 ABI bug wrt handling of 128-bit bit-fields

Status in QEMU:
  Fix Released

Bug description:
  After upgrading to Ubuntu 20.04 LTS, GCC 9.3 displays a lot of notes:

  hw/block/pflash_cfi01.c: In function ‘pflash_mem_read_with_attrs’:
  hw/block/pflash_cfi01.c:663:20: note: parameter passing for argument of type 
‘MemTxAttrs’ {aka ‘struct MemTxAttrs’} changed in GCC 9.1
    663 | static MemTxResult pflash_mem_read_with_attrs(void *opaque, hwaddr 
addr, uint64_t *value,
        |                    ^~~~~~~~~~~~~~~~~~~~~~~~~~
  hw/block/pflash_cfi01.c: In function ‘pflash_mem_write_with_attrs’:
  hw/block/pflash_cfi01.c:677:20: note: parameter passing for argument of type 
‘MemTxAttrs’ {aka ‘struct MemTxAttrs’} changed in GCC 9.1
    677 | static MemTxResult pflash_mem_write_with_attrs(void *opaque, hwaddr 
addr, uint64_t value,
        |                    ^~~~~~~~~~~~~~~~~~~~~~~~~~~
  hw/nvram/fw_cfg.c: In function ‘fw_cfg_dma_mem_valid’:
  hw/nvram/fw_cfg.c:475:13: note: parameter passing for argument of type 
‘MemTxAttrs’ {aka ‘struct MemTxAttrs’} changed in GCC 9.1
    475 | static bool fw_cfg_dma_mem_valid(void *opaque, hwaddr addr,
        |             ^~~~~~~~~~~~~~~~~~~~
  hw/nvram/fw_cfg.c: In function ‘fw_cfg_data_mem_valid’:
  hw/nvram/fw_cfg.c:483:13: note: parameter passing for argument of type 
‘MemTxAttrs’ {aka ‘struct MemTxAttrs’} changed in GCC 9.1
    483 | static bool fw_cfg_data_mem_valid(void *opaque, hwaddr addr,
        |             ^~~~~~~~~~~~~~~~~~~~~
  hw/nvram/fw_cfg.c: In function ‘fw_cfg_ctl_mem_valid’:
  hw/nvram/fw_cfg.c:501:13: note: parameter passing for argument of type 
‘MemTxAttrs’ {aka ‘struct MemTxAttrs’} changed in GCC 9.1
    501 | static bool fw_cfg_ctl_mem_valid(void *opaque, hwaddr addr,
        |             ^~~~~~~~~~~~~~~~~~~~
  hw/nvram/fw_cfg.c: In function ‘fw_cfg_comb_valid’:
  hw/nvram/fw_cfg.c:521:13: note: parameter passing for argument of type 
‘MemTxAttrs’ {aka ‘struct MemTxAttrs’} changed in GCC 9.1
    521 | static bool fw_cfg_comb_valid(void *opaque, hwaddr addr,
        |             ^~~~~~~~~~~~~~~~~
  hw/intc/arm_gic.c: In function ‘gic_do_hyp_read’:
  hw/intc/arm_gic.c:1996:20: note: parameter passing for argument of type 
‘MemTxAttrs’ {aka ‘struct MemTxAttrs’} changed in GCC 9.1
   1996 | static MemTxResult gic_do_hyp_read(void *opaque, hwaddr addr, 
uint64_t *data,
        |                    ^~~~~~~~~~~~~~~
  hw/intc/arm_gic.c: In function ‘gic_thiscpu_hyp_read’:
  hw/intc/arm_gic.c:1979:20: note: parameter passing for argument of type 
‘MemTxAttrs’ {aka ‘struct MemTxAttrs’} changed in GCC 9.1
   1979 | static MemTxResult gic_thiscpu_hyp_read(void *opaque, hwaddr addr, 
uint64_t *data,
        |                    ^~~~~~~~~~~~~~~~~~~~
  hw/intc/arm_gic.c: In function ‘gic_get_current_pending_irq’:
  hw/intc/arm_gic.c:419:17: note: parameter passing for argument of type 
‘MemTxAttrs’ {aka ‘struct MemTxAttrs’} changed in GCC 9.1
    419 | static uint16_t gic_get_current_pending_irq(GICState *s, int cpu,
        |                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~

  This seems related to:
  https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88469
  https://gcc.gnu.org/git/?p=gcc.git&a=commit;h=c590597c45

    This is pretty unlikely in real code, but similar to Arm, the AArch64
    ABI has a bug with the handling of 128-bit bit-fields, where if the
    bit-field dominates the overall alignment the back-end code may end up
    passing the argument correctly.  This is a regression that started in
    gcc-6 when the ABI support code was updated to support overaligned
    types.  The fix is very similar in concept to the Arm fix.  128-bit
    bit-fields are fortunately extremely rare, so I'd be very surprised if
    anyone has been bitten by this.

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