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X-OriginatorOrg: arm.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 03 Sep 2025 11:00:13.5382 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: 906075f1-df12-4d47-339a-08ddead90ef9 X-MS-Exchange-CrossTenant-Id: f34e5979-57d9-4aaa-ad4d-b122a662184d X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=f34e5979-57d9-4aaa-ad4d-b122a662184d;Ip=[4.158.2.129];Helo=[outbound-uk1.az.dlp.m.darktrace.com] X-MS-Exchange-CrossTenant-AuthSource: DB1PEPF000509F9.eurprd02.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: DB3PR08MB9010 Hi Dave, [...] > > > > > > .Lskip_indirection: > > > > > > .Lskip_tcr2: > > > > > > + mrs_s x1, SYS_ID_AA64MMFR3_EL1 > > > > > > + ubfx x1, x1, #ID_AA64MMFR3_EL1_SCTLRX_SHIFT, #4 > > > > > > + cbz x1, .Lskip_sctlr2 > > > > > > + mrs_s x1, SYS_SCTLR2_EL12 > > > > > > + msr_s SYS_SCTLR2_EL1, x1 > > > > > > > > > > > > + // clean SCTLR2_EL1 > > > > > > + mov_q x1, INIT_SCTLR2_EL1 > > > > > > + msr_s SYS_SCTLR2_EL12, x1 > > > > > > > > > > I'm still not sure why we need to do this. The code doesn't seem to > > > > > clean up by the EL1 value of any other register -- or have I missed > > > > > something? > > > > > > > > > > We have already switched to EL2, via the HVC call that jumped to > > > > > __finalise_el2. We won't run at EL1 again unless KVM starts a guest -- > > > > > but in that case, it's KVM's responsibility to set up the EL1 registers > > > > > before handing control to the guest. > > > > > > > > > > In any case, is SCTLR2_EL1 ever set to anything except INIT_SCTLR2_EL1 > > > > > before we get here? > > [...] > > > When I look at init_el2(), it returns to EL1 via: > > > > mov x0, #INIT_PSTATE_EL1 > > msr spsr_el2, x0 > > ... > > eret > > > > In other words, from init_kernel_el() through finalise_el2(), > > all system-register accesses are made at EL1 (i.e., SYS_REG_EL1). > > During this period, it appears that only SCTLR_EL1 is modified, > > so the code only needs to care about the accessed register — SCTLR_EL1. > > > > That’s why SCTLR_EL1 is reinitialised at the end of finalise_el2(). > > Otherwise, the MMU bit might remain enabled, which could cause errors later > > when launching a VM under VHE. > > > > However, the idea behind this patch is to initialise SCTLR2_ELx > > the same way as SCTLR_ELx. > > I’m not sure whether SCTLR2_ELx is modified during this period. > > If it is (now or in the future), > > it should be cleared/reinitialised just like SCTLR_EL1. > > > > This patch is based on the assumption that there may be modifications to > > SCTLR2_ELx during this period. So it isn’t about other system registers; > > it’s about the register actually used during this period. > > > > Am I missing anything? > > > > Thanks! > > > > -- > > Sincerely, > > Yeoreum Yun > > I think I missed the SCTLR_EL1 reset in the idmap code after the > enter_vhe label. > > Actually, I'm not sure whether there is any architectural reason for > setting SCTLR_EL1 to INIT_SCTLR_EL1_MMU_OFF here. "for good measure" > suggests that it felt like a good idea but there was no known reason > for it. The commit message for the original patch doesn't offer an > explanation -- maybe Marc can remember. > > This might be a defence against speculative translation table walks > using the EL1&0 regime (but the architecture says [RNRJPP]: "If an > implementation is executing at EL3 or EL2, the PE is not permitted to > use the registers associated with the EL1&0 translation regime to > speculatively access memory or translation tables.") So it shouldn't > really matter, but in case buggy CPUs don't implement this rule > properly it may be a good idea to turn the stage1 MMU off just in case. > Thanks for great deep insight :D. > Since it's there, though, it probably does make sense to reinitialise > SCTLR2_EL1 at the same time -- but can you move this so that it is next > to the SCTLR_EL1 reinitialisation? Otherwise, the purpose of > reinitialising SCTLR2_EL1 is unclear. It really should come under the > same "for good measure" justification as the SCTLR_EL1 reset. Okay. > > However, I don't think this has anything to do with putting things into > a clean state for VMs. KVM defines the reset state for all the _EL1 > regs explicitly -- failing to do that would be a bug in KVM. > > (See arch/arm64/kvm/sys_regs.c : sys_reg_descs[], kvm_reset_sys_regs().) Right. I've missed the reset sysregs when kvm is launching. Thanks! -- Sincerely, Yeoreum Yun