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X-OriginatorOrg: amd.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 20 Jan 2026 14:41:55.6114 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: 1a9d4e15-ecc7-41f2-0db7-08de58320f08 X-MS-Exchange-CrossTenant-Id: 3dd8961f-e488-4e60-8e11-a82d994e183d X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=3dd8961f-e488-4e60-8e11-a82d994e183d;Ip=[165.204.84.17];Helo=[satlexmb07.amd.com] X-MS-Exchange-CrossTenant-AuthSource: CY4PEPF0000EE37.namprd05.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: PH0PR12MB8798 On Tue Jan 20, 2026 at 2:29 PM CET, Jan Beulich wrote: > On 20.01.2026 14:11, Teddy Astie wrote: >> Le 20/01/2026 =C3=A0 10:56, Alejandro Vallejo a =C3=A9crit=C2=A0: >>> --- a/xen/arch/x86/hvm/svm/vmcb.c >>> +++ b/xen/arch/x86/hvm/svm/vmcb.c >>> @@ -66,6 +66,12 @@ static int construct_vmcb(struct vcpu *v) >>> GENERAL2_INTERCEPT_XSETBV | GENERAL2_INTERCEPT_ICEBP = | >>> GENERAL2_INTERCEPT_RDPRU; >>> =20 >>> + if ( cpu_has_bus_lock_thresh ) >>> + { >>> + vmcb->_general3_intercepts =3D GENERAL3_INTERCEPT_BUS_LOCK_THR= ESH; >>> + vmcb->bus_lock_thresh =3D 1; /* trigger immediately */ >>> + } >>> + >>> /* Intercept all debug-register writes. */ >>> vmcb->_dr_intercepts =3D ~0u; >>> =20 >>=20 >> According to APM, >>=20 >> INTERCEPT_BUS_LOCK_THRESH does >> > Intercept bus lock operations when Bus Lock Threshold Counter is 0 >>=20 >> I assume that when set to 0, we intercept all bus locks, so if set to 1,= =20 >> every 2 bus lock (since we first go from 1 to 0, then at 0 we intercept= =20 >> the next one) ? Correct >>=20 >> I think we want that to be tunable, as intercepting all bus locks may be= =20 >> too extreme we probably want to intercept every few ones instead. > > Otoh bus locks (as opposed to cache locks) would better be rare, or else > perhaps such a guest deserves some extra slowing down? > > Jan I agree with Jan. buslock operations are so rare it's not worth being lenie= nt about them. They ought to be more expensive than they currently are for the caller, in fact, so they notice and not do it because there's rarely a rati= onale for them to exist at all. I'd be happier if the ISA didn't allow them at al= l. Cheers, Alejandro