From: Oliver Upton Date: Fri, 12 Sep 2025 21:22:52 +0000 (-0700) Subject: KVM: arm64: nv: Expose FEAT_ECBHB to NV-enabled VMs X-Git-Url: https://www.infradead.org/git/?a=commitdiff_plain;h=7cbdb25bed4046dacf139cce25fad9ef39a04a5f;p=users%2Fhch%2Fmisc.git KVM: arm64: nv: Expose FEAT_ECBHB to NV-enabled VMs The exact wording of the restrictions on branch prediction due to FEAT_ECBHB in DDI0487L.b is as follows: When FEAT_ECBHB is implemented, the branch history information created in a context before an exception to a higher Exception level using AArch64 cannot be used by code before that exception to exploitatively control the execution of any indirect branches in code in a different context after the exception. While vEL2 and EL1 are multiplexed at EL1, they exist in different hardware-described contexts as KVM uses different stage-2 MMUs to represent the corresponding translation regimes. Additionally, exception entries into vEL2 always imply a hardware exception entry into literal EL2 for the emulated regime change. Given all of this, and the fact that FEAT_ECBHB places no limitation on the EL of the protected context after the exception, we can claim FEAT_ECBHB on supporting hardware. Signed-off-by: Oliver Upton Signed-off-by: Marc Zyngier --- diff --git a/arch/arm64/kvm/nested.c b/arch/arm64/kvm/nested.c index 35fa6e00c9be..20e7b11d5d67 100644 --- a/arch/arm64/kvm/nested.c +++ b/arch/arm64/kvm/nested.c @@ -1524,8 +1524,7 @@ u64 limit_nv_id_reg(struct kvm *kvm, u32 reg, u64 val) break; case SYS_ID_AA64MMFR1_EL1: - val &= ~(ID_AA64MMFR1_EL1_ECBHB | - ID_AA64MMFR1_EL1_CMOW | + val &= ~(ID_AA64MMFR1_EL1_CMOW | ID_AA64MMFR1_EL1_TIDCP1 | ID_AA64MMFR1_EL1_nTLBPA | ID_AA64MMFR1_EL1_AFP |