x86/kvm: fix FASTOP_SIZE when return thunks are enabled

The return thunk call makes the fastop functions larger, just like IBT
does. Consider a 16-byte FASTOP_SIZE when CONFIG_RETHUNK is enabled.

Otherwise, functions will be incorrectly aligned and when computing their
position for differently sized operators, they will executed in the middle
or end of a function, which may as well be an int3, leading to a crash
like:

[   36.091116] int3: 0000 [#1] SMP NOPTI
[   36.091119] CPU: 3 PID: 1371 Comm: qemu-system-x86 Not tainted 5.15.0-41-generic #44
[   36.091120] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.15.0-1 04/01/2014
[   36.091121] RIP: 0010:xaddw_ax_dx+0x9/0x10 [kvm]
[   36.091185] Code: 00 0f bb d0 c3 cc cc cc cc 48 0f bb d0 c3 cc cc cc cc 0f 1f 80 00 00 00 00 0f c0 d0 c3 cc cc cc cc 66 0f c1 d0 c3 cc cc cc cc <0f> 1f 80 00 00 00 00 0f c1 d0 c3 cc cc cc cc 48 0f c1 d0 c3 cc cc
[   36.091186] RSP: 0018:ffffb1f541143c98 EFLAGS: 00000202
[   36.091188] RAX: 0000000089abcdef RBX: 0000000000000001 RCX: 0000000000000000
[   36.091188] RDX: 0000000076543210 RSI: ffffffffc073c6d0 RDI: 0000000000000200
[   36.091189] RBP: ffffb1f541143ca0 R08: ffff9f1803350a70 R09: 0000000000000002
[   36.091190] R10: ffff9f1803350a70 R11: 0000000000000000 R12: ffff9f1803350a70
[   36.091190] R13: ffffffffc077fee0 R14: 0000000000000000 R15: 0000000000000000
[   36.091191] FS:  00007efdfce8d640(0000) GS:ffff9f187dd80000(0000) knlGS:0000000000000000
[   36.091192] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   36.091192] CR2: 0000000000000000 CR3: 0000000009b62002 CR4: 0000000000772ee0
[   36.091195] PKRU: 55555554
[   36.091195] Call Trace:
[   36.091197]  <TASK>
[   36.091198]  ? fastop+0x5a/0xa0 [kvm]
[   36.091222]  x86_emulate_insn+0x7b8/0xe90 [kvm]
[   36.091244]  x86_emulate_instruction+0x2f4/0x630 [kvm]
[   36.091263]  ? kvm_arch_vcpu_load+0x7c/0x230 [kvm]
[   36.091283]  ? vmx_prepare_switch_to_host+0xf7/0x190 [kvm_intel]
[   36.091290]  complete_emulated_mmio+0x297/0x320 [kvm]
[   36.091310]  kvm_arch_vcpu_ioctl_run+0x32f/0x550 [kvm]
[   36.091330]  kvm_vcpu_ioctl+0x29e/0x6d0 [kvm]
[   36.091344]  ? kvm_vcpu_ioctl+0x120/0x6d0 [kvm]
[   36.091357]  ? __fget_files+0x86/0xc0
[   36.091362]  ? __fget_files+0x86/0xc0
[   36.091363]  __x64_sys_ioctl+0x92/0xd0
[   36.091366]  do_syscall_64+0x59/0xc0
[   36.091369]  ? syscall_exit_to_user_mode+0x27/0x50
[   36.091370]  ? do_syscall_64+0x69/0xc0
[   36.091371]  ? syscall_exit_to_user_mode+0x27/0x50
[   36.091372]  ? __x64_sys_writev+0x1c/0x30
[   36.091374]  ? do_syscall_64+0x69/0xc0
[   36.091374]  ? exit_to_user_mode_prepare+0x37/0xb0
[   36.091378]  ? syscall_exit_to_user_mode+0x27/0x50
[   36.091379]  ? do_syscall_64+0x69/0xc0
[   36.091379]  ? do_syscall_64+0x69/0xc0
[   36.091380]  ? do_syscall_64+0x69/0xc0
[   36.091381]  ? do_syscall_64+0x69/0xc0
[   36.091381]  entry_SYSCALL_64_after_hwframe+0x61/0xcb
[   36.091384] RIP: 0033:0x7efdfe6d1aff
[   36.091390] Code: 00 48 89 44 24 18 31 c0 48 8d 44 24 60 c7 04 24 10 00 00 00 48 89 44 24 08 48 8d 44 24 20 48 89 44 24 10 b8 10 00 00 00 0f 05 <41> 89 c0 3d 00 f0 ff ff 77 1f 48 8b 44 24 18 64 48 2b 04 25 28 00
[   36.091391] RSP: 002b:00007efdfce8c460 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
[   36.091393] RAX: ffffffffffffffda RBX: 000000000000ae80 RCX: 00007efdfe6d1aff
[   36.091393] RDX: 0000000000000000 RSI: 000000000000ae80 RDI: 000000000000000c
[   36.091394] RBP: 0000558f1609e220 R08: 0000558f13fb8190 R09: 00000000ffffffff
[   36.091394] R10: 0000558f16b5e950 R11: 0000000000000246 R12: 0000000000000000
[   36.091394] R13: 0000000000000001 R14: 0000000000000000 R15: 0000000000000000
[   36.091396]  </TASK>
[   36.091397] Modules linked in: isofs nls_iso8859_1 kvm_intel joydev kvm input_leds serio_raw sch_fq_codel dm_multipath scsi_dh_rdac scsi_dh_emc scsi_dh_alua ipmi_devintf ipmi_msghandler drm msr ip_tables x_tables autofs4 btrfs blake2b_generic zstd_compress raid10 raid456 async_raid6_recov async_memcpy async_pq async_xor async_tx xor raid6_pq libcrc32c raid1 raid0 multipath linear crct10dif_pclmul crc32_pclmul ghash_clmulni_intel aesni_intel virtio_net net_failover crypto_simd ahci xhci_pci cryptd psmouse virtio_blk libahci xhci_pci_renesas failover
[   36.123271] ---[ end trace db3c0ab5a48fabcc ]---
[   36.123272] RIP: 0010:xaddw_ax_dx+0x9/0x10 [kvm]
[   36.123319] Code: 00 0f bb d0 c3 cc cc cc cc 48 0f bb d0 c3 cc cc cc cc 0f 1f 80 00 00 00 00 0f c0 d0 c3 cc cc cc cc 66 0f c1 d0 c3 cc cc cc cc <0f> 1f 80 00 00 00 00 0f c1 d0 c3 cc cc cc cc 48 0f c1 d0 c3 cc cc
[   36.123320] RSP: 0018:ffffb1f541143c98 EFLAGS: 00000202
[   36.123321] RAX: 0000000089abcdef RBX: 0000000000000001 RCX: 0000000000000000
[   36.123321] RDX: 0000000076543210 RSI: ffffffffc073c6d0 RDI: 0000000000000200
[   36.123322] RBP: ffffb1f541143ca0 R08: ffff9f1803350a70 R09: 0000000000000002
[   36.123322] R10: ffff9f1803350a70 R11: 0000000000000000 R12: ffff9f1803350a70
[   36.123323] R13: ffffffffc077fee0 R14: 0000000000000000 R15: 0000000000000000
[   36.123323] FS:  00007efdfce8d640(0000) GS:ffff9f187dd80000(0000) knlGS:0000000000000000
[   36.123324] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   36.123325] CR2: 0000000000000000 CR3: 0000000009b62002 CR4: 0000000000772ee0
[   36.123327] PKRU: 55555554
[   36.123328] Kernel panic - not syncing: Fatal exception in interrupt
[   36.123410] Kernel Offset: 0x1400000 from 0xffffffff81000000 (relocation range: 0xffffffff80000000-0xffffffffbfffffff)
[   36.135305] ---[ end Kernel panic - not syncing: Fatal exception in interrupt ]---

Fixes: aa3d4803 ("x86: Use return-thunk in asm code")
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
Co-developed-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Borislav Petkov <bp@suse.de>
Cc: Josh Poimboeuf <jpoimboe@kernel.org>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Reported-by: Linux Kernel Functional Testing <lkft@linaro.org>
Message-Id: <20220713171241.184026-1-cascardo@canonical.com>
Tested-by: Jack Wang <jinpu.wang@ionos.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>