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X-MS-Exchange-CrossTenant-Id: 4e2c6054-71cb-48f1-bd6c-3a9705aca71b X-MS-Exchange-CrossTenant-MailboxType: HOSTED X-MS-Exchange-CrossTenant-UserPrincipalName: omhJk+BR16TtzCLg0mL4b8S2XgYbKH2ApVfrh+DlVT2lsFPWaRicWRGERDCMozRvvjvlZLKZbCpiGz4UP+RLFtj9Zxhub1Wnv225zB4pXRk= X-MS-Exchange-Transport-CrossTenantHeadersStamped: PH0PR10MB7061 X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1117,Hydra:6.1.9,FMLib:17.12.80.40 definitions=2025-09-25_01,2025-09-24_01,2025-03-28_01 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 adultscore=0 phishscore=0 bulkscore=0 spamscore=0 mlxlogscore=999 mlxscore=0 suspectscore=0 malwarescore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2508110000 definitions=main-2509250122 X-Authority-Analysis: v=2.4 cv=fd2ty1QF c=1 sm=1 tr=0 ts=68d53de5 cx=c_pps a=XiAAW1AwiKB2Y8Wsi+sD2Q==:117 a=XiAAW1AwiKB2Y8Wsi+sD2Q==:17 a=6eWqkTHjU83fiwn7nKZWdM+Sl24=:19 a=z/mQ4Ysz8XfWz/Q5cLBRGdckG28=:19 a=lCpzRmAYbLLaTzLvsPZ7Mbvzbb8=:19 a=wKuvFiaSGQ0qltdbU6+NXLB8nM8=:19 a=Ol13hO9ccFRV9qXi2t6ftBPywas=:19 a=xqWC_Br6kY4A:10 a=IkcTkHD0fZMA:10 a=yJojWOMRYYMA:10 a=GoEa3M9JfhUA:10 a=0oqpFwLfRYPdpCEDvRcA:9 a=3ZKOabzyN94A:10 a=QEXdDO2ut3YA:10 X-Proofpoint-ORIG-GUID: vHV6tGZwpvTHbQDT5cpdEMgeThPr1gD9 X-Proofpoint-Spam-Details-Enc: AW1haW4tMjUwOTIwMDAzNCBTYWx0ZWRfX9SbswvGTiegi btAUK46eqNdtscFe3rn9RG9hF8xpZkBW3diiMxMHwkGr8xfqRnCis3PWBWMHO/HW/xt2lw9XtOv Il8K7FsQxWTNgM9ygPvLOZDnJGnN7+XG80wi4nXg0gqEovNN6Gy6bF4nWblYKv2yO5jos8WU84U J8SOAkbQfVlcKdQhfn/wIQI2hpDYrMkskufd0kFKLTOjePJWVxB8JAzMn5/RsubWWLXjI+1qyB4 dawYKj2SMY3kvlmwamlekRxucLjfvxvqnj0ujAUE26K1DqSSQ1mAyxSuogtWumDsLA84j8ObQAS 5TJq6oxmEOv7yb1sOceLI0VF/Q1//VD6/n5jhhEsdIfXce3+S8Y4hSpCWJGoxZ7Lb0q/D2G6xXV 55ucSeXX X-Proofpoint-GUID: vHV6tGZwpvTHbQDT5cpdEMgeThPr1gD9 On 9/25/25 14:43, Peter Zijlstra wrote: > On Thu, Sep 25, 2025 at 12:29:18PM +0100, Andrew Cooper wrote: >>> I've made that: >>> >>> case 0x90: >>> if (rex_b) /* XCHG %r8, %rax */ >>> break; >>> >>> if (prefix == 0xf3) /* REP NOP := PAUSE */ >>> break; >>> >>> insn->type = INSN_NOP; >>> break; >> >> Legacy prefixes can come in any order.  What is F3 66 90 ? >> >> Also, VEX/EVEX/REX2 want excluding too, all of which can encode rex_b >> differently. > > So luckily objtool only really cares about instructions as found in the > kernel text. Neither f3 66 90 nor VEX/EVEX/REX2 prefixes are of much > concern. And it looks like objtool ignores VEX instructions earlier in the same function: int arch_decode_instruction(struct objtool_file *file, const struct section *sec, unsigned long offset, unsigned int maxlen, struct instruction *insn) { ... if (ins.vex_prefix.nbytes) return 0; ... } vex_prefix is set for VEX/EVEX/VEX3/VEX2. alex. > But yes.. I happen to have an insn_is_nop() function that can be used on > userspace, and that certainly wants to be taught about these... x86 is > such a pain :/ > >> Is it really only rex_b which prevents NOP becoming a pause, or is it >> any REX prefix?  I would have thought it was any REX prefix. > > SDM opcode table and instruction reference seems consistent with f3 > only.