[Bug gdb/30818] New: GDB + GDBserver doesn't set AVX512 registers properly

["simon.marchi at polymtl dot ca" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=30818

            Bug ID: 30818
           Summary: GDB + GDBserver doesn't set AVX512 registers properly
           Product: gdb
           Version: HEAD
            Status: NEW
          Severity: normal
          Priority: P2
         Component: gdb
          Assignee: unassigned at sourceware dot org
          Reporter: simon.marchi at polymtl dot ca
  Target Milestone: ---

On a machine with AVX512 support (AMD EPYC 9634), I see:

$ make check TESTS="gdb.arch/i386-avx512.exp"
RUNTESTFLAGS="--target_board=native-gdbserver"
...
FAIL: gdb.arch/i386-avx512.exp: check contents of zmm_data[16] after writing
ZMM regs
...

I can reproduce more simply with (after connecting to gdbserver):

(gdb) print $zmm16.v8_int64 
$1 = {0, 0, 0, 0, 0, 0, 0, 0}
(gdb) print $zmm16.v8_int64 = {11,22,33,44,55,66,77,88}
$2 = {11, 22, 33, 44, 55, 66, 77, 88}
(gdb) print $zmm16.v8_int64
$3 = {11, 22, 33, 44, 55, 66, 77, 88}
(gdb) step
5               ++x;
(gdb) print $zmm16.v8_int64
$4 = {11, 22, 77, 88, 0, 0, 0, 0}

It looks like writing to the local regcache in GDB worked fine, but the
writeback to gdbserver didn't work fine.

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