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-Signal handlers in x86_64 userspace have wrongly aligned stack
-Description of problem:
-Various applications crash in signal handlers due to `movaps` getting a misaligned stack address. For some reason this is reported as a NULL deref, but `gdb` clearly shows the true cause.
-
-```plaintext
-> qemu-x86_64 /usr/bin/ruby -e '`true`'
--e:1: [BUG] Segmentation fault at 0x0000000000000000
-ruby 3.2.2 (2023-03-30 revision e51014f9c0) [x86_64-linux-gnu]
-
--- Control frame information -----------------------------------------------
-c:0003 p:---- s:0011 e:000010 CFUNC  :`
-c:0002 p:0005 s:0006 e:000005 EVAL   -e:1 [FINISH]
-c:0001 p:0000 s:0003 E:0015b0 DUMMY  [FINISH]
-
--- Ruby level backtrace information ----------------------------------------
--e:1:in `<main>'
--e:1:in ``'
-
--- Machine register context ------------------------------------------------
- RIP: 0x00002aaaab50f98a RBP: 0x00002aaaabb136b8 RSP: 0x00002aaaab2a9c98
- RAX: 0x0000000000000000 RBX: 0x0000000000004946 RCX: 0x0000000000000000
- RDX: 0x00002aaaab2a9c98 RDI: 0x000000000caf0000 RSI: 0x0000000000000000
-  R8: 0x00002aaaab2aaa50  R9: 0x0000000000000050 R10: 0x0000000000000008
- R11: 0x0000000000000000 R12: 0x0000000000000002 R13: 0x0000000000007310
- R14: 0x0000000000005e10 R15: 0x00002aaab0537f20 EFL: 0x0000000000000246
-
--- C level backtrace information -------------------------------------------
-```
-
-```plaintext
-(gdb) x/i $pc
-=> 0x2aaaab50f98a:      movaps %xmm0,(%rsp)
-(gdb) p/x $rsp
-$3 = 0x2aaaab2a9998
-```
-Steps to reproduce:
-1. ```qemu-x86_64 /usr/bin/ruby -e '`true`'```
-Additional information:
-The x86_64 psABI says:
-
-> the value (%rsp − 8) is always a multiple of 16 when control is transferred to the function entry point.
-
-However, when QEMU jumps to the signal handler, $rsp is aligned to 16B, i.e. ends in `0x..0`.
-
-The relevant kernel code:
-
-https://elixir.bootlin.com/linux/v6.5.5/source/arch/x86/kernel/signal.c#L123
-
-```plaintext
-	sp -= frame_size;
-
-	if (ia32_frame)
-		/*
-		 * Align the stack pointer according to the i386 ABI,
-		 * i.e. so that on function entry ((sp + 4) & 15) == 0.
-		 */
-		sp = ((sp + 4) & -FRAME_ALIGNMENT) - 4;
-	else
-		sp = round_down(sp, FRAME_ALIGNMENT) - 8;
-```
-
-CC @lvivier @bonzini @rth7680