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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 3.0 on 10.30.177.17 On 5/12/26 16:32, Paolo Bonzini wrote: The trace shows that CET is not used at all unless MBEC is > present. In particular (after "trace-cmd record -e kvm ...") I can do: > > $ trace-cmd report |grep -e msr_write.*da0| sed 's/.*kvm_/kvm_/' | sort -u > > and it shows as expected this with +vmx-mbec,+cet-ss,+cet-ibt: > > kvm_msr: msr_write da0 = 0x800 > > but not with -vmx-mbec,+cet-ss,+cet-ibt. This initialization is > performed by Hyper-V even before VMXON, and the breakage happens even > if Memory Integrity is disabled inside Windows. > > Knowing that Hyper-V was not running any nested guest at the time of > the hang, I changed __vmcs_writel() to have > > if (field == SECONDARY_VM_EXEC_CONTROL) value &= > ~SECONDARY_EXEC_MODE_BASED_EPT_EXEC; I have now reproduced the guest hang with a one line change on top of kvm/master: diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c index 937aeb474af7..43e0f20e4e26 100644 --- a/arch/x86/kvm/vmx/nested.c +++ b/arch/x86/kvm/vmx/nested.c @@ -7231,6 +7231,7 @@ static void nested_vmx_setup_secondary_ctls(u32 ept_caps, if (enable_ept) { /* nested EPT: emulate EPT also to L1 */ msrs->secondary_ctls_high |= + SECONDARY_EXEC_MODE_BASED_EPT_EXEC | /* hem hem */ SECONDARY_EXEC_ENABLE_EPT; msrs->ept_caps = VMX_EPT_PAGE_WALK_4_BIT | (which would break very badly if Hyper-V were to start a nested guest, but the trace says it doesn't). Can you check what behavior you get from this (actually silly) change? It should allow you to exercise Hyper-V's CET paths without the burden of the MMU changes. Paolo