From 4422d8b7502db1b910c519763135a99836efb5e5 Mon Sep 17 00:00:00 2001 From: Marc Zyngier Date: Thu, 30 Jul 2026 08:09:56 +0100 Subject: [PATCH] KVM: arm64: Merge guest's HCRX_EL2 using NV_HCRX_GUEST_EXCLUDE The way we merge the guest-provided HCRX_EL2 value with the host's is bonkers. We try to make it look like the FGT registers by using positive and negative polarities for traps, but most of these bits are not strictly about trapping, as they actively change the way some architectural state is managed. It would be far better to deal with these bits like we do for HCR_EL2, by enumerating the list of bits we don't allow the guest to override. This is simplified by the fact that HCRX_EL2 only affects EL1, and not EL2. Re-jig the HCRX_EL2 handling with a macro that list the bits excluded from the merge (TMEA, PTTWI, EnIDCP128). Reviewed-by: Yuan Yao Signed-off-by: Marc Zyngier Reviewed-by: Joey Gouly Link: https://patch.msgid.link/20260730071022.296811-4-maz@kernel.org Signed-off-by: Oliver Upton --- arch/arm64/kvm/hyp/include/hyp/switch.h | 22 ++++++++++++++++++++-- 1 file changed, 20 insertions(+), 2 deletions(-) diff --git a/arch/arm64/kvm/hyp/include/hyp/switch.h b/arch/arm64/kvm/hyp/include/hyp/switch.h index 18131e395e24..0903e08f4b6a 100644 --- a/arch/arm64/kvm/hyp/include/hyp/switch.h +++ b/arch/arm64/kvm/hyp/include/hyp/switch.h @@ -325,6 +325,24 @@ static inline void __deactivate_traps_mpam(void) write_sysreg_s(MPAMHCR_HOST_FLAGS, SYS_MPAMHCR_EL2); } +/* + * Just like for HCR_EL2, we can't let the guest mess with some of the + * basics we rely on in HCRX_EL2. However, the major difference is that + * HCRX_EL2 only affects EL1, and never EL2 (sudden outburst of sanity, I + * guess). So it is always the guest inflicting it on its own guestx. + * + * Things we don't want to let the guest control are: + * + * - TMEA: That's for us to decide how an SEA is routed, not the guest. + * + * - PTTWI: Similarly, it is for us to decide whether Reduced Coherency for + * the PTW is a thing. It really isn't. + * + * - EnIDCP128: We don't allow IMPDEF sysregs -- full stop. + */ +#define NV_HCRX_GUEST_EXCLUDE (HCRX_EL2_TMEA | HCRX_EL2_PTTWI | \ + HCRX_EL2_EnIDCP128) + static inline void __activate_traps_common(struct kvm_vcpu *vcpu) { struct kvm_cpu_context *hctxt = host_data_ptr(host_ctxt); @@ -350,8 +368,8 @@ static inline void __activate_traps_common(struct kvm_vcpu *vcpu) u64 hcrx = vcpu->arch.hcrx_el2; if (is_nested_ctxt(vcpu)) { u64 val = __vcpu_sys_reg(vcpu, HCRX_EL2); - hcrx |= val & __HCRX_EL2_MASK; - hcrx &= ~(~val & __HCRX_EL2_nMASK); + hcrx |= (val & ~NV_HCRX_GUEST_EXCLUDE); + hcrx &= ~(~val & ~NV_HCRX_GUEST_EXCLUDE); } ctxt_sys_reg(hctxt, HCRX_EL2) = read_sysreg_s(SYS_HCRX_EL2);